Note: This page contains sample records for the topic gas phase nucleation from Science.gov.
While these samples are representative of the content of Science.gov,
they are not comprehensive nor are they the most current set.
We encourage you to perform a real-time search of Science.gov
to obtain the most current and comprehensive results.
Last update: November 12, 2013.
1

Kinetic theory of steady chemical nucleation in the gas phase  

Microsoft Academic Search

We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the effective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in

T. Yamamoto; T. Chigai; S. Watanabe; T. Kozasa

2001-01-01

2

Kinetic theory of steady chemical nucleation in the gas phase  

NASA Astrophysics Data System (ADS)

We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the effective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in a multi-component vapor, in which nucleation occurs via the chemical reactions yielding a condensate having a stoichiometric composition. An exact expression of the steady nucleation rate is given together with its approximate formulas for practical applications. The present formulation is not concerned with any particular cluster model. The supersaturation ratio for a many-component vapor is defined so as to be a natural extension of that for a one-component vapor. It is shown that the transition probabilities due to growth and decay of the clusters are of the same form as the growth and evaporation rates in a one-component vapor.

Yamamoto, T.; Chigai, T.; Watanabe, S.; Kozasa, T.

2001-12-01

3

Heterogeneous nucleation of a monomer gas on a growing gas-phase polymer  

NASA Astrophysics Data System (ADS)

We present a new theory for the nucleation of gas-phase monomers in a supersaturated vapor on a growing short-chain polymer. Reiss and co-workers have shown that cloud chambers can be extraordinarily sensitive detectors of single molecules of particular size in such experiments, giving valuable information about rate constants for gas-phase polymerization. We have used a density functional approach to calculate the free energy and heterogeneous nucleation rate of liquid drops of monomer on oligomer chains of varying length. We show that the free energy is not a simple monotonic function of chain length as assumed in simple classical theories.

Seok, Chaok; Oxtoby, David W.

1999-05-01

4

Gas Hydrate Nucleation Processes  

NASA Astrophysics Data System (ADS)

The onset of gas hydrate nucleation is greatly affected by the thermal history of the water that forms its lattice structure. Hydrate formation experiments were performed in a 72 liter pressure vessel by bubbling carbon dioxide through a 1 liter column at hydrate formation pressures (1.4 to 3.7 MPa) and temperatures (275.0 to 278.0 K) to quantify this effect. They show that when even a fraction ( e. g. 20 %) of the water in which hydrate has formed was recently frozen and thawed, the overpressurization for nucleation was reduced by an average of 50 % versus experiments performed in distilled water. In those experiments where a lower overpressure is present when hydrate nucleated, they tended to form on the surface of bubbles, whereas when a higher amount of overpressure was necessary for hydrate to nucleate, they appeared to form abruptly on bubble surfaces as well as from the bulk liquid phase. In approximation of classical nucleation, hydrate formation could be described as occurring by the spontaneous joining together of arising components of the hydrate lattice. In water that was frozen, and kept at a low temperature (< 275 K), molecular simulation models predict the predominance of water molecules organized as penatmeters, a possible subunit of the hydrate lattice. Our results suggest that in nature, initiation of hydrate formation may be strongly influenced by temperature dependant pre-structuring of water molecules prior to their contact with gas.

David, R. E.; Zatsepina, O.; Phelps, T. J.

2003-12-01

5

THE NUCLEATION AND GROWTH OF GAS BUBBLES IN A NEWTONIAN FLUID: AN ENERGETIC VARIATIONAL PHASE FIELD APPROACH  

Microsoft Academic Search

In this paper, we study the nucleation and growth of gas bubbles in a Newtonian fluid. We employ a general energetic variational formulation with a phase-field method, and compare the analytical and numerical predictions of this new formulation with those of classical models. The new approach allows the study of bubble nucleation, growth and coalescence in a unified framework, and

AARON NABER; CHUN LIU; JAMES J. FENG

6

Pore-scale interfacial dynamics during gas-supersaturated water injection in porous media - on nucleation, growth and advection of disconnected fluid phases (Invited)  

NASA Astrophysics Data System (ADS)

Degassing and in situ development of a mobile gas bubbles occur when injecting supersaturated aqueous phase into water-saturated porous media. Supersaturated water injection (SWI) has potentially significant applications in remediation of soils contaminated by non-aqueous phase liquids and in enhanced oil recovery. Pore network simulations indicate the formation of a region near the injection boundary where gas phase nuclei are activated and grow by mass transfer from the flowing supersaturated aqueous phase. Ramified clusters of gas-filled pores develop which, owing to the low prevailing Bond number, grow laterally to a significant extent prior to the onset of mobilization, and are thus likely to coalesce. Gas cluster mobilization invariably results in fragmentation and stranding, such that a macroscopic region containing few tenuously connected large gas clusters is established. Beyond this region, gas phase nucleation and mass transfer from the aqueous phase are limited by diminishing supply of dissolved gas. New insights into SWI dynamics are obtained using rapid micro-visualization in transparent glass micromodels. Using high-speed imaging, we observe the nucleation, initial growth and subsequent fate (mobilization, fragmentation, collision, coalescence and stranding) of CO2 bubbles and clusters of gas-filled pores and analyze cluster population statistics. We find significant support for the development of invasion-percolation-like patterns, but also report on hitherto unaccounted for gas bubble behavior. Additionally, we report for the first time on the acoustic emission signature of SWI in porous media and relate it to the dynamics of bubble nucleation and growth. Finally, we identify the pore-scale mechanisms associated with the mobilization and subsequent recovery of a residual non-aqueous phase liquid due to gas bubble dynamics during SWI.

Or, D.; Ioannidis, M.

2010-12-01

7

Formation of Protein Condensed Phases: Nucleation Mechanisms.  

PubMed

Proteins in solution form a number of condensed phases. Even omitting the amyloid structures formed after partial protein unfolding, these phases include crystals, polymers, and other solid aggregates, as well as dense liquids and gels. Some of these condensed phases underlie pathological conditions, others play a crucial role in the biological function of the respective protein or are an essential part of its laboratory or industrial processing. In this review, we summarize the fundamentals and recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We define the transition from nucleation to spinodal decomposition for these two phase transitions. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. The concepts and mechanisms reviewed here provide powerful tools for control of the nucleation process by varying the solution thermodynamic parameters. PMID:22489787

Vekilov, Peter G

2012-04-01

8

Nucleation of Ordered Phases in Block Copolymers  

NASA Astrophysics Data System (ADS)

Nucleation of various ordered phases in block copolymers is studied by examining the free-energy landscape within the self-consistent field theory. The minimum energy path (MEP) connecting two ordered phases is computed using a recently developed string method. The shape, size, and free-energy barrier of critical nuclei are obtained from the MEP, providing information about the emergence of a stable ordered phase from a metastable phase. In particular, structural evolution of embryonic gyroid nucleus is predicted to follow two possible MEPs, revealing an interesting transition pathway with an intermediate perforated layered structure.

Cheng, Xiuyuan; Lin, Ling; E, Weinan; Zhang, Pingwen; Shi, An-Chang

2010-04-01

9

Nucleation of ordered phases in block copolymers.  

PubMed

Nucleation of various ordered phases in block copolymers is studied by examining the free-energy landscape within the self-consistent field theory. The minimum energy path (MEP) connecting two ordered phases is computed using a recently developed string method. The shape, size, and free-energy barrier of critical nuclei are obtained from the MEP, providing information about the emergence of a stable ordered phase from a metastable phase. In particular, structural evolution of embryonic gyroid nucleus is predicted to follow two possible MEPs, revealing an interesting transition pathway with an intermediate perforated layered structure. PMID:20481967

Cheng, Xiuyuan; Lin, Ling; E, Weinan; Zhang, Pingwen; Shi, An-Chang

2010-04-09

10

Asymptotic evaluation of probability density functions for mean aerosol particle formation rates by homogeneous nucleation in turbulent gas jets  

Microsoft Academic Search

Mean aerosol particle formation rates resulting from homogeneous nucleation in turbulent gas jets are evaluated using a probability density function (pdf) for temperature and vapor concentration fluctuations in the gas phase. Nucleation rates are assumed to depend sensitively on the gas-phase conditions, allowing asymptotic analysis to be used for evaluating mean particle formation rates. The asymptotic analysis shows that to

Benjamin D. Shaw

2004-01-01

11

Analysis of urban gas phase ammonia measurements from the 2002 Atlanta Aerosol Nucleation and Real-Time Characterization Experiment (ANARChE)  

NASA Astrophysics Data System (ADS)

Gas phase ammonia (NH3) measurements were made in July and August 2002 during the Atlanta Aerosol Nucleation and Real-Time Characterization Experiment with two different chemical ionization mass spectrometry techniques. Correlations between the 1 min data from both instruments yielded a slope of 1.17 and an intercept of -0.295 ppbv, with a linear correlation coefficient (r2) of 0.71. Ambient NH3 mixing ratios ranged from 0.4 to 13 ppbv. NH3 observations were compared to the Community Multiscale Air Quality (CMAQ) modeling system as well as a thermodynamic equilibrium model, ISORROPIA, used by CMAQ to predict NH3 partitioning. A morning rise in both observed and modeled NH3 mixing ratios strongly suggests a regional influence due to automobile emissions. However, at midday the predicted NH3 decreased to less than 0.5 ppbv, while the observations remained around 3 ppbv. Both observed and modeled ammonium nitrate levels were too low to support the observed midday NH3 mixing ratios. ISORROPIA calculations of NH3 constrained by the total measured ammonia mass (NH3 + ammonium (NH4+)) agreed well with the observations (slope of 1.25 and r2 of 0.75). For times when the net aerosol charge was near zero the agreement was excellent (slope of 1.22 and r2 of 0.88). These results indicate that for most of the observed conditions, ISORROPIA could accurately predict NH3 partitioning. The observations suggest that local sunlight- or temperature-driven NH3 sources, such as soil emissions, may be responsible for the discrepancy between the model results and measured values.

Nowak, J. B.; Huey, L. G.; Russell, A. G.; Tian, D.; Neuman, J. A.; Orsini, D.; Sjostedt, S. J.; Sullivan, A. P.; Tanner, D. J.; Weber, R. J.; Nenes, A.; Edgerton, E.; Fehsenfeld, F. C.

2006-09-01

12

Gas-liquid nucleation in associating fluids  

NASA Astrophysics Data System (ADS)

The density functional approach to nucleation is extended to the study of the condensation of associating fluids. We use Wertheim theory for associating liquids in a system of spherical interacting molecules with single association sites. Our calculations show that classical nucleation theory largely underestimates the height of the nucleation barrier in this kind of system, but most of that error can be corrected through the classical model of Katz, Saltsburg, and Reiss for nucleation in associated vapors.

Talanquer, V.; Oxtoby, David W.

2000-01-01

13

Pore-scale interfacial dynamics during gas-supersaturated water injection in porous media - on nucleation, growth and advection of disconnected fluid phases (Invited)  

Microsoft Academic Search

Degassing and in situ development of a mobile gas bubbles occur when injecting supersaturated aqueous phase into water-saturated porous media. Supersaturated water injection (SWI) has potentially significant applications in remediation of soils contaminated by non-aqueous phase liquids and in enhanced oil recovery. Pore network simulations indicate the formation of a region near the injection boundary where gas phase nuclei are

D. Or; M. Ioannidis

2010-01-01

14

Nucleation of gas hydrates within constant energy systems.  

PubMed

The early stage of formation of gas hydrates has recently attracted attention as amorphous intermediate gas hydrate structures have been observed, apparently contrary to a classical model of nucleation and some experimental observations. To date, essentially all reported molecular simulations of the nucleation of gas hydrates have been under constant temperature conditions, which does not consider the possible impacts of heat transfer on the nucleation processes. Here we show, using constant energy molecular simulations, that the nuclei at an early stage of the hydrate formation have relatively more crystalline order in comparison with those observed in previous isothermal (NPT or NVT) work. The current work suggests a more transient role for intermediate amorphous structures during hydrate nucleation, thereby providing a stronger link between molecular simulation and experimental observations. Our NVE results nevertheless support the two-step nucleation mechanism proposed in previous simulation studies under constant temperature conditions which features the initial formation of amorphous hydrate-like structures. PMID:23330680

Liang, Shuai; Kusalik, Peter G

2013-01-29

15

Nucleation and growth of Nb nanoclusters during plasma gas condensation  

NASA Astrophysics Data System (ADS)

Niobium nanoclusters were produced using a plasma gas condensation process. The influence of gas flow rate, aggregation length, and source current on the nanocluster nucleation and growth were analyzed. Nanoclusters with an average diameter from 4 nm to 10 nm were produced. Cluster size and concentration were tuned by controlling the process inputs. The effects of each parameter on the nucleation zone, growth length, and residence time was examined. The parameters do not affect the cluster formation and growth independently; their influence on cluster formation can be either cumulative or competing. Examining the nucleation and growth over a wide combination of parameters provided insight into their interactions and the impact on the growth process. These results provide the opportunity for a broader understanding into the nucleation and growth of nanoclusters and some insights into how process parameters interact during deposition. This knowledge will enhance the ability to create nanoclusters with desired size dispersions.

Bray, K. R.; Jiao, C. Q.; DeCerbo, J. N.

2013-06-01

16

Nucleation and growth studies of crystalline carbon phases at nanoscale  

Microsoft Academic Search

Understanding the nucleation and early stage growth of crystals from the vapor phase is important for realizing large-area single-crystal quality films, controlled synthesis of nanocrystals, and the possible discovery of new phases of materials. Carbon provides the most interesting system because all its known crystalline phases (diamond, graphite and carbon nanotubes) are technologically important materials. Hence, this dissertation is focused

Radhika C. Mani

2004-01-01

17

Gas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tube  

Microsoft Academic Search

During the observation of HCFC141b gas hydrate growth processes outside a vertical heat transfer tube, two exciting phenomena\\u000a were found: fast nucleation of gas hydrate from melting ice, and the spontaneous permeation of water into the guest phases\\u000a along the surface of heat transfer tube to form gas hydrate continuously. These two phenomena were explained with Zhou & Sloan’s\\u000a hypothesis

Yingming Xie; Kaihua Guo; Deqing Liang; Shuanshi Fan; Jianming Gu; Jinggui Chen

2005-01-01

18

Nucleation  

PubMed Central

Crystallization starts with nucleation and control of nucleation is crucial for the control of the number, size, perfection, polymorphism and other characteristics of crystalline materials. This is particularly true for crystallization in solution, which is an essential part of processes in the chemical and pharmaceutical industries and a major step in physiological and pathological phenomena. There have been significant recent advances in the understanding of the mechanism of nucleation of crystals in solution. The foremost of these are the two-step mechanism of nucleation and the notion of the solution–crystal spinodal. According to the two-step mechanism, the crystalline nucleus appears inside pre-existing metastable clusters of size several hundred nanometers, which consist of dense liquid and are suspended in the solution. While initially proposed for protein crystals, the applicability of this mechanism has been demonstrated for small molecule organic materials, colloids, polymers, and biominerals. This mechanism helps to explain several long-standing puzzles of crystal nucleation in solution: nucleation rates which are many orders of magnitude lower than theoretical predictions, the significance of the dense protein liquid, and others. At high supersaturations typical of most crystallizing systems, the generation of crystal embryos occurs in the spinodal regime, where the nucleation barrier is negligible. The solution-crystal spinodal helps to understand the role of heterogeneous substrates in nucleation and the selection of crystalline polymorphs. Importantly, these ideas provide powerful tools for control of the nucleation process by varying the solution thermodynamic parameters.

Vekilov, Peter G.

2010-01-01

19

Adiabatic nucleation in the liquid-vapor phase transition  

NASA Astrophysics Data System (ADS)

The fundamental difference between classical (isothermal) nucleation theory (CNT) and adiabatic nucleation theory (ANT) is discussed. CNT uses the concept of isothermal heterophase fluctuations, while ANT depends on common fluctuations of the thermodynamic variables. Applications to the nonequilibrium liquid to vapor transition are shown. However, we cannot yet calculate nucleation frequencies. At present, we can only indicate at what temperatures and pressures copious homogeneous nucleation is expected in the liquid to vapor phase transition. It is also explained why a similar general indication cannot be made for the inverse vapor to liquid transition. Simultaneously, the validity of Peng-Robinson's equation of state [D.-Y. Peng and D. B. Robinson, Ind. Eng. Chem. Fundam. 15, 59 (1976)] is confirmed for highly supersaturated liquids.

de Sá, Elon M.; Meyer, Erich; Soares, Vitorvani

2001-05-01

20

Gas bubble nucleation and growth in cohesive sediments  

Microsoft Academic Search

Sediment often contains a significant amount of organic material, which can be decomposed by bacterial activity. During this process and under anaerobic conditions that prevail in sediments, mainly methane and carbon dioxide are formed. These compounds will dissolve in the pore water, until the level of saturation is attained.Experiments show that gas bubble nucleation occurs already at a small oversaturation

Walther van Kesteren; Thijs van Kessel

2002-01-01

21

Nucleation and growth studies of crystalline carbon phases at nanoscale  

NASA Astrophysics Data System (ADS)

Understanding the nucleation and early stage growth of crystals from the vapor phase is important for realizing large-area single-crystal quality films, controlled synthesis of nanocrystals, and the possible discovery of new phases of materials. Carbon provides the most interesting system because all its known crystalline phases (diamond, graphite and carbon nanotubes) are technologically important materials. Hence, this dissertation is focused on studying the nucleation and growth of carbon phases synthesized from the vapor phase. Nucleation experiments were performed in a microwave plasma chemical vapor deposition (CVD) reactor, and the resulting carbon nanocrystals were analyzed primarily using electron nanodiffraction and Raman spectroscopy. These studies led to the discovery of two new crystalline phases of sp 3 carbon other than diamond: face-centered and body-centered cubic carbon. Nanodiffraction results revealed possible hydrogen substitution into diamond-cubic lattices, indicating that these new phases probably act as intermediates in diamond nucleation. Nucleation experiments also led to the discovery of two new morphologies for sp2 carbon: nanocrystals of graphite and tapered, hollow 1-D structures termed here as "carbon nanopipettes". A Kinetic Monte Carlo (KMC) algorithm was developed to simulate the growth of individual diamond crystals from the vapor phase, starting with small clusters of carbon atoms (or seeds). Specifically, KMC simulations were used to distinguish the kinetic rules that give rise to a star-shaped decahedral morphology compared to decahedral crystals. KMC simulations revealed that slow adsorption on the {111} step-propagation sites compared to kink sites leads to star-decahedral crystals, and higher adsorption leads to decahedral crystals. Since the surfaces of the nanocrystals of graphite and nanopipettes were expected to be composed primarily of edge-plane sites, the electrochemical behavior of both these materials were investigated with compounds requiring chemisorption, specifically biologically important species. Both these materials exhibited a stable and reversible voltammetric behavior for dopamine (a neurotransmitter) similar to that of graphite edge planes. Furthermore, a simple bottom-up concept utilizing the tapered morphology of the nanopipettes was developed to assemble a nanoarray sensor for fast cyclic voltammetry. In summary, the main outcomes of this dissertation include: the discovery of new crystalline carbon phases, understanding kinetic faceting of multiply twinned diamond crystals and tapered morphologies of carbon nanotubes, and development of new electrode materials based on sp2 carbon nanocrystals for sensing biologically important analytes.

Mani, Radhika C.

22

Magnetostatic Effects in the Nucleation of Rare Earth Ferromagnetic Phases  

SciTech Connect

It has been reported that superheating, supercooling, and explosive kinetics coupled to other degrees of freedom occur at the ferromagnetic transitions of Er and Dy, and that metastable phases occur during the transition kinetics of Er. We explain these observations in terms of magnetostatic energy, which requires highly eccentric nuclei in the homogeneous nucleation of magnetic transitions in heavy rare earths. The magnetostatics favor transitions through ferrimagnetic intermediaries. The unusual kinetics derive from effective spin lattice relaxation.

Durfee, C. S.; Flynn, C. P.

2001-07-30

23

Kinetics of the initial stage of isothermal gas phase formation  

Microsoft Academic Search

at least one nucleus bubble and of the total volume V, of the newly formed gas phase in the case of progressive or instantaneous bubble nucleation. Different definitions of the critical supersaturation for gas phase formation are considered and their experimental applicability is discussed. The general results are applied to the particular cases of gas phase formation at constant and

D. Kashchiev; A. Firoozabadi

1993-01-01

24

Ice nucleation properties of the most abundant mineral dust phases  

NASA Astrophysics Data System (ADS)

The ice nucleation properties of the nine most abundant minerals occurring in desert aerosols (quartz, albite, microcline, kaolinite, montmorillonite, illite, calcite, gypsum, and hematite) were investigated by environmental scanning electron microscopy (ESEM). In this instrument, the pure minerals are exposed to water vapor at variable pressures and temperatures. The crystallization of ice on the mineral particles is observed by secondary electron imaging, and the supersaturation for an activated particle fraction of 1-3% is determined as function of temperature. In all experiments, condensation of water prior to ice formation was not observed within detectable limits, even at water supersaturation. The highest temperatures for 1-3% activation vary between -10°C and -16°C for the nine minerals investigated, and the corresponding onset relative humidities relative to ice RHi between 107 and 117%. Supersaturation temperature curves for initial ice formation (1-3% activation) in the temperature range typical for mixed-phase clouds were measured for all nine minerals. The temperature dependence of the onset relative humidity is strongly dependent on mineralogy. Kaolinite, montmorillonite, and hematite show a strong increase in RHi with decreasing temperature, whereas RHi is almost constant for illite, albite, quartz, and calcite. The highly variable ice nucleation properties of the various mineral dust components should be considered for parameterization schemes. Illite and kaolinite are the most important minerals to consider, as they have high ice nucleation efficiency and are common components of desert aerosols.

Zimmermann, Frank; Weinbruch, Stephan; Schütz, Lothar; Hofmann, Heiko; Ebert, Martin; Kandler, Konrad; Worringen, Annette

2008-12-01

25

Phase field approach to heterogeneous crystal nucleation in alloys  

NASA Astrophysics Data System (ADS)

We extend the phase field model of heterogeneous crystal nucleation developed recently [L. Gránásy , Phys. Rev. Lett. 98, 035703 (2007)] to binary alloys. Three approaches are considered to incorporate foreign walls of tunable wetting properties into phase field simulations: a continuum realization of the classical spherical cap model (called model A herein), a nonclassical approach (model B) that leads to ordering of the liquid at the wall and to the appearance of a surface spinodal, and a nonclassical model (model C) that allows for the appearance of local states at the wall that are accessible in the bulk phases only via thermal fluctuations. We illustrate the potential of the presented phase field methods for describing complex polycrystalline solidification morphologies including the shish-kebab structure, columnar to equiaxed transition, and front-particle interaction in binary alloys.

Warren, James A.; Pusztai, Tamás; Környei, László; Gránásy, László

2009-01-01

26

Nucleation of allotropic phases during pulsed laser annealing of manganese  

SciTech Connect

Manganese has four allotropes with an equilibrium melting point of the high temperature delta-phase at 1517/sup 0/K and calculated metastable melting points for the ..gamma.., ..beta.. and ..cap alpha.. phases at 1501/sup 0/K, 1481/sup 0/K and 1395/sup 0/K, respectively. Our observations for Mn irradiated with a pulsed laser and supporting estimates of maximum allotropic transition rates indicate that transformations between allotropes are suppressed during heating with approx.25 ns laser pulses, as well as during subsequent cooling. Upon pulsed heating of ..cap alpha..-Mn to the melt threshold, the melt is undercooled 122/sup 0/K below the delta-Mn melting point. For incident laser pulse energy densities near the melting threshold, resolidification involves regrowth of ..cap alpha..-Mn from the substrate. At energy densities well above threshold, the ..gamma..-Mn phase forms by separate nucleation and growth from the undercooled melt, and is retained upon rapid solidification. We conclude that significant melt undercooling, which may exceed 100/sup 0/K, can occur during pulsed laser melting of metallic crystals and that the resulting crystalline structure is determined by both thermodynamics and nucleation kinetics.

Perepezko, J.H.; Follstaedt, D.M.; Peercy, P.S.

1985-01-01

27

Nucleation of the Solid Phase by Cavitation in an Undercooled Liquid which expands on Freezing  

Microsoft Academic Search

HICKLING1 has proposed that nucleation by cavitation in an undercooled liquid which expands on freezing is caused by adiabatic compression into a region where a high pressure phase is stable: the high pressure phase nucleates and later transforms to the stable low pressure phase. Originally, his arguments were intended to explain only the results on water, but were later extended

J. D. Hunt; K. A. Jackson

1966-01-01

28

Control of protein crystal nucleation around the metastable liquid-liquid phase boundary  

PubMed Central

The capability to enhance or suppress the nucleation of protein crystals opens opportunities in various fundamental and applied areas, including protein crystallography, production of protein crystalline pharmaceuticals, protein separation, and treatment of protein condensation diseases. Herein, we show that the rate of homogeneous nucleation of lysozyme crystals passes through a maximum in the vicinity of the liquid–liquid phase boundary hidden below the liquidus (solubility) line in the phase diagram of the protein solution. We found that glycerol and polyethylene glycol (which do not specifically bind to proteins) shift this phase boundary and significantly suppress or enhance the crystal nucleation rates, although no simple correlation exists between the action of polyethylene glycol on the phase diagram and the nucleation kinetics. The control mechanism does not require changes in the protein concentration, acidity, and ionicity of the solution. The effects of the two additives on the phase diagram strongly depend on their concentration, which provides opportunities for further tuning of nucleation rates.

Galkin, Oleg; Vekilov, Peter G.

2000-01-01

29

Monte Carlo tests of nucleation concepts in the lattice gas model  

NASA Astrophysics Data System (ADS)

The conventional theory of homogeneous and heterogeneous nucleation in a supersaturated vapor is tested by Monte Carlo simulations of the lattice gas (Ising) model with nearest-neighbor attractive interactions on the simple cubic lattice. The theory considers the nucleation process as a slow (quasistatic) cluster (droplet) growth over a free energy barrier ?F*, constructed in terms of a balance of surface and bulk term of a critical droplet of radius R*, implying that the rates of droplet growth and shrinking essentially balance each other for droplet radius R=R*. For heterogeneous nucleation at surfaces, the barrier is reduced by a factor depending on the contact angle. Using the definition of physical clusters based on the Fortuin-Kasteleyn mapping, the time dependence of the cluster size distribution is studied for quenching experiments in the kinetic Ising model and the cluster size ?* where the cluster growth rate changes sign is estimated. These studies of nucleation kinetics are compared to studies where the relation between cluster size and supersaturation is estimated from equilibrium simulations of phase coexistence between droplet and vapor in the canonical ensemble. The chemical potential is estimated from a lattice version of the Widom particle insertion method. For large droplets it is shown that the physical clusters have a volume consistent with the estimates from the lever rule. Geometrical clusters (defined such that each site belonging to the cluster is occupied and has at least one occupied neighbor site) yield valid results only for temperatures less than 60% of the critical temperature, where the cluster shape is nonspherical. We show how the chemical potential can be used to numerically estimate ?F* also for nonspherical cluster shapes.

Schmitz, Fabian; Virnau, Peter; Binder, Kurt

2013-05-01

30

Nucleation, kinetics and morphology of displacive phase transformations in iron  

NASA Astrophysics Data System (ADS)

An extensive, systematic molecular dynamics (MD) study is performed for analysing the nucleation, kinetics and morphology characteristics of thermally-induced, displacive phase transformations from face-centred cubic (fcc) to body-centred cubic (bcc) iron. At the atomic level these transformation characteristics are influenced by a number of factors, including (i) the appearance of free surfaces, (ii) the initial presence of fcc-bcc grain boundaries, (iii) the existence of point defects (i.e., atomic vacancies) near a grain boundary, (iv) the initial thermal velocities of the atoms, and (v) the specific interatomic potential used. Other MD studies that capture the overall transformation behaviour of iron well have often underestimated or ignored the influence by these factors on the transformation response, with the risk of putting the accuracy, generality and physical explanation of the MD results on loose grounds. The present research illustrates the relative contribution of each of the above factors by means of a detailed comparison study for three different interatomic potentials. The accuracy of the interatomic potentials is established by validating for the fcc and bcc phases the calculated elastic moduli, cohesive energy, vacancy formation energy and interfacial energy against experimental and ab initio data reported in the literature. The importance of calibrating material data of both the stable bcc phase and the metastable fcc phase - instead of the stable bcc phase only - is demonstrated. The numerical results call for general caution when interpreting phenomena that start close to instability points and therefore are sensitive to small disturbances; a large spread in the overall transformation time is found under different initial thermal velocities, interfacial lattice incoherence, boundary conditions (free vs. periodic), and interatomic potentials, where for completely transformed atomic systems the discrepancy between the maximum and minimum transformation time appears to be more than a factor of 150. The transformation time is phenomenologically related to the overall activation energy and the cohesive energy difference of the fcc and bcc phases, which, beyond a certain combination of values, may even prevent the transformation process from occurring. Also, the morphology of the bcc product phase is remarkably sensitive to the type of boundary conditions and the choice of interatomic potential, while the influence by both the set of initial thermal velocities and the interfacial lattice incoherence only becomes apparent for specific atomic samples that transform relatively slowly. The presence of fcc-bcc grain boundaries increases the spatial heterogeneity of transformation events, with the appearance of an increasing number of vacancies at the grain boundary giving rise to a larger overall transformation time. The 10 main results following from the present MD study are conveniently summarised at the end of this communication.

Suiker, A. S. J.; Thijsse, B. J.

2013-11-01

31

Measurements of Ice Nucleation in Arctic Mixed Phase Clouds  

NASA Astrophysics Data System (ADS)

Here we present in-flight measurements of ice nuclei (IN) data collected with a Continuous Flow Diffusion Chamber (CFDC) from onboard the Canadian Convair during the recent Indirect and Semidirect Effects of Aerosol Campaign (ISDAC). During ISDAC, Arctic haze containing aerosols from several distinctly different sources including Asian dust and Siberia coal fire plants, were sampled. The corresponding range in aerosol-cloud interactions and overall influence of outside aerosol masses on the Arctic atmosphere was variable as well. During the project, ice nuclei were sampled in-cloud and out-of-cloud and in the presence of multiple distinct aerosol layers. The CFDC was operated at controlled temperatures in the range of -10 degrees C to -40 degrees C and over a wide range of supersaturations with respect to ice. Observed IN concentrations varied from frequent values of 0.01 per liter to spikes as high as approximately 10 per liter during Arctic haze events. The multiple factors contributing to this high degree of variability in ice nuclei will be discussed in an effort to determine what drives ice nucleation in mixed phase clouds in the Arctic.

Brooks, S. D.; Glen, A.; Zelenyuk, A. N.; MacDonald, A. M.; Liu, P.; Leaitch, R.

2008-12-01

32

Study of Gypsum Crystal Nucleation and Growth Rates in Simulated Flue Gas Desulfurization Liquors. Final Report.  

National Technical Information Service (NTIS)

The kinetics of gypsum crystal nucleation and growth rates were measured in flue gas desulfurization (FGD) scrubber liquors. Variables studied were parent seed crystal size and mass; the organic additives citric acid, adipic acid, sodium dodecylbenzene su...

A. D. Randolph D. Etherton

1981-01-01

33

Phase transitions mediated by the nucleation of single vortices in a superconducting channel  

NASA Astrophysics Data System (ADS)

We study the nucleation of single vortices in a 2D channel in a superconducting/superfluid state which connects two bulk samples. Using real space renormalization group techniques, we show that boundary effects associated with the nucleation of single vortices drives the system to a phase transition at a temperature lower than the Berezinsk?–Kosterlitz–Thouless temperature.

Sobnack, M. B.; Kusmartsev, F. V.

2013-10-01

34

Thermodynamics and Kinetics of Nanoclusters Controlling Gas-to-Particle Nucleation  

SciTech Connect

Nucleation of new particles from vapor-phase molecular precursors is an important process in the synthesis of nanomaterials and in the formation of aerosols in the atmosphere. Vapor-to-particle nucleation is a macroscopic process controlled by nanoscale particles (e.g., molecular clusters). Computational approaches to nucleation have been limited by the lack of a consistent theory of the process and by the lack of efficient approaches to simulate the properties of clusters relevant to nucleation. In this article, we focus on two advances that allow nucleation to be treated in a rigorous manner for molecular systems: Dynamical Nucleation Theory permits a consistent treatment of the nucleation kinetics and aggregation-volume-bias Monte Carlo simulations using self-adaptive umbrella sampling combined with histogram reweighting provides an efficient approach to evaluate the thermodynamics of molecular clusters important in nucleation. The combination of these two approaches positions molecular computational approaches to make significant advances in our understanding of the mechanisms of nucleation, particularly in multiple component systems that play crucial roles in nanoscience applications and in the atmosphere. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Kathmann, Shawn M.; Schenter, Gregory K.; Garrett, Bruce C.; Chen, Bin; Siepmann, J. I.

2009-06-18

35

Multiscale approach to CO2 hydrate formation in aqueous solution: phase field theory and molecular dynamics. Nucleation and growth.  

PubMed

A phase field theory with model parameters evaluated from atomistic simulations/experiments is applied to predict the nucleation and growth rates of solid CO(2) hydrate in aqueous solutions under conditions typical to underwater natural gas hydrate reservoirs. It is shown that under practical conditions a homogeneous nucleation of the hydrate phase can be ruled out. The growth rate of CO(2) hydrate dendrites has been determined from phase field simulations as a function of composition while using a physical interface thickness (0.85+/-0.07 nm) evaluated from molecular dynamics simulations. The growth rate extrapolated to realistic supersaturations is about three orders of magnitude larger than the respective experimental observation. A possible origin of the discrepancy is discussed. It is suggested that a kinetic barrier reflecting the difficulties in building the complex crystal structure is the most probable source of the deviations. PMID:16821944

Tegze, György; Pusztai, Tamás; Tóth, Gyula; Gránásy, László; Svandal, Atle; Buanes, Trygve; Kuznetsova, Tatyana; Kvamme, Bjorn

2006-06-21

36

Characterization of Heterogeneously Nucleated Phase Transformations and Characterization of Associated Microstructures.  

National Technical Information Service (NTIS)

Homogeneous and grain boundary heterogeneous nucleation and growth phase transformation processes have been shown to follow four distinct types of kinetic behavior. Kinetics are determined by processing treatment, interfacial and grain boundary energies, ...

J. E. Sanchez

1976-01-01

37

Propandiol vapor nucleation rates  

NASA Astrophysics Data System (ADS)

Consideration of vapor-gas nucleation as binary vapor nucleation (instead widely used the one component approximation for nucleation of this system now) may lead the progress in the development of nucleation theory. Observations of phase transitions initiated by the carrier gas in the critical embryos of condensate can be a sufficiently convincing argument in this discussion. In order to confirm the role of the carrier gases received in the recent research1, in present study 1,2-propanediol and 1,3-propanediol vapor nucleation rates were measured. Carbon dioxide (Tc=304.2 K,Pc=7.39 MPa) and sulfur hexafluoride (Tc=318.7 K,Pc=3.75 MPa) were chosen as the carrier gases, because of their low and convenient critical temperatures, Tc, and critical pressures, Pc. Analysis of the experimental data shows that gas-carrier molecules are involved in new phase embryo formation. Vapor nucleation of investigated substances in a carrier gas atmosphere can be considered as nucleation of binary system. .

Anisimov, M. P.; Nasibulin, A. G.; Timoshina, L. V.; Koropchak, J. A.

2000-08-01

38

1,2-propanediol and 1,3-propanediol homogeneous nucleation rates and phase transitions in the new phase critical embryos  

NASA Astrophysics Data System (ADS)

Nucleation rates of supersaturated vapors near the conditions for the critical line for each pair of 1,2- or 1,3-propanediol and carbon dioxide or sulfur hexafluoride at total system pressures of P=0.10, 0.20, and 0.30 MPa have been measured in a flow diffusion chamber. Critical parameters, i.e., temperatures and pressures, of the binary systems versus compositions were semiempirically evaluated. It was found experimentally that there is an area of parameters in the PTx phase diagram, for each pair of vapor and carrier gas investigated in the present research, for which critical embryo phase transitions of the first order take place (where P, T, and x are pressure, temperature, and embryo composition, respectively). The nucleation rate surface singularity and a gap in the number of molecules in critical embryos reflect this area. Shifts of the phase transition temperatures can be initiated by increasing the pressure (or concentration) of the carrier gas. This behavior is peculiar for binary systems. It is well-known that no liquid-liquid phase transitions can exist for a one-component system. At least a binary solution is required for liquid-liquid phase transitions. This means that vapor nucleation of the investigated substances in the carrier gas atmosphere can be considered as nucleation of a binary vapor-gas system. A schematic diagram for the possible metastable vapor conditions is presented. On the basis of our experimental results, one can assume that there exists a surface describing the parameters related with a phase transition of the first order in critical embryos. This surface is located within the volume of the possible vapor metastable conditions.

Anisimov, M. P.; Koropchak, J. A.; Nasibulin, A. G.; Timoshina, L. V.

2000-06-01

39

The effect of magma flow on nucleation of gas bubbles in a volcanic conduit  

NASA Astrophysics Data System (ADS)

We solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously, which allow us to predict bubble sizes and number densities under ascent conditions. As magma rises toward the surface, the pressure decreases and eventually becomes less than the solubility pressure. When the degree of supersaturation becomes great enough, bubbles nucleate. Nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases. We show that a second nucleation event may occur just below the fragmentation level. Near that level, the degree of supersaturation continuously increases as the magma is rapidly decompressed. As a result, nucleation will not stop until fragmentation occurs. This second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices. The bubbles of the second nucleation event have high internal gas pressures up to 2 MPa greater than the liquid pressure, suggesting that the second nucleation event may enhance fragmentation of magma. We apply the model to the calculation protocol defined at the “Volcanic eruption mechanism modeling workshop, Durham, 2002”. We found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing.

Massol, Hélène; Koyaguchi, Takehiro

2005-05-01

40

A systematic investigation of the "Nucleation Phase" of large global earthquakes using broadband teleseismic data  

NASA Astrophysics Data System (ADS)

Earthquake's dynamic motion starts suddenly but often with an interval of relatively weak motion, which was coined the "nucleation phase" by Ellsworth and Beroza (1995). Ellsworth and Beroza (1995, 1996) found that the nucleation phase existed in the near-source records of all 41 earthquakes they studied, spanning a magnitude range from M 1 to M 8. On average the nucleation phase accounts for ~0.5% of the total moment but exists for about 1/6 of the total duration. Ji et al. (2010) investigated the initiations of 19 Mw 8.0 earthquakes since 1994 using a new approach applied to teleseismic broadband data. They found that about fifty percent of the earthquakes had a weak initiation phase; the durations of the weak initiation phases are consistent with the dataset based on near-source records in terms of their correlation with final seismic moments. We use the same method to study the rupture initiation of 60 Mw 7.5-8.0 earthquakes since 1994. Our preliminary result indicates that the rate at which weak nucleations are observed is about the same as that for Mw 8.0 earthquakes. Interestingly we more often observe the weak nucleation phases in strike-slip earthquakes than thrust or normal earthquakes. Second, we developed a method to study the moment magnitude of nucleation phases by matching the stacked waveforms with 1D synthetic seismograms. Our preliminary result indicates that the 4 s nucleation phase associated with the recent 2011 Mw 9.1 Tohoku earthquake had a moment magnitude of 5.4 -a value inconsistent with the previous observations.

Burkhart, E.; Ji, C.

2011-12-01

41

Nucleation of ordered solid phases of proteins via a disordered high-density state: Phenomenological approach  

NASA Astrophysics Data System (ADS)

Nucleation of ordered solid phases of proteins triggers numerous phenomena in laboratory, industry, and in healthy and sick organisms. Recent simulations and experiments with protein crystals suggest that the formation of an ordered crystalline nucleus is preceded by a disordered high-density cluster, akin to a droplet of high-density liquid that has been observed with some proteins; this mechanism allowed a qualitative explanation of recorded complex nucleation kinetics curves. Here, we present a simple phenomenological theory that takes into account intermediate high-density metastable states in the nucleation process. Nucleation rate data at varying temperature and protein concentration are reproduced with high fidelity using literature values of the thermodynamic and kinetic parameters of the system. Our calculations show that the growth rate of the near-critical and supercritical ordered clusters within the dense intermediate is a major factor for the overall nucleation rate. This highlights the role of viscosity within the dense intermediate for the formation of the ordered nucleus. The model provides an understanding of the action of additives that delay or accelerate nucleation and presents a framework within which the nucleation of other ordered protein solid phases, e.g., the sickle cell hemoglobin polymers, can be analyzed.

Pan, Weichun; Kolomeisky, Anatoly B.; Vekilov, Peter G.

2005-05-01

42

Role of nucleation and growth in two-phase microstructure formation  

NASA Astrophysics Data System (ADS)

During the directional solidification of peritectic alloys, a rich variety of two-phase microstructures develop, and the selection process of a specific microstructure is complicated due to the following two considerations. (1) In contrast to many single phase and eutectic microstructures that grow under steady state conditions, two-phase microstructures in a peritectic system often evolve under non-steady-state conditions that can lead to oscillatory microstructures, and (2) the microstructure is often governed by both the nucleation and the competitive growth of the two phases in which repeated nucleation can occur due to the change in the local conditions during growth. In this research, experimental studies in the Sn-Cd system were designed to isolate the effects of nucleation and competitive growth on the dynamics of complex microstructure formation. Experiments were carried out in capillary samples to obtain diffusive growth conditions so that the results can be analyzed quantitatively. At high thermal gradient and low velocity, oscillatory microstructures were observed in which repeated nucleation of the two phases was observed at the wall-solid-liquid junction. Quantitative measurements of nucleation undercooling were obtained for both the primary and the peritectic phase nucleation, and three different ampoule materials were used to examine the effect of different contact angles at the wall on nucleation undercooling. Nucleation undercooling for each phase was found to be very small, and the experimental undercooling values were orders of magnitude smaller than that predicted by the classical theory of nucleation. A new nucleation mechanism is proposed in which the clusters of atoms at the wall ahead of the interface can become a critical nucleus when the cluster encounters the triple junction. Once the nucleation of a new phase occurs, the microstructure is found to be controlled by the relative growth of the two phases that give rise to different oscillatory microstructures that depend on the imposed velocity and the size of the sample. At low thermal gradient to velocity ratio, a steady-state composite microstructure is observed. Two mechanisms of composite microstructure formation were examined: (i) the formation of the peritectic phase in the intercellular region of the primary phase where the solute rejected by the primary phase is absorbed by the peritectic phase. The peritectic phase forms a small distance behind the growing primary phase front. (ii) The second mechanism is the coupled growth of the two phases with a macroscopically planar interface, as in the case of eutectic growth. Detailed studies showed that this composite microstructure, although it appears as a eutectic microstructure, did not grow in the coupled manner at the advancing interface in the Sn-Cd system. However, a new observation was made when experiments were carried out in thin ampoule of Ta. The peritectic phase nucleated at the wall-interface triple junction and grew along the wall, while the primary phase continued to grow at the center, giving rise to a steady-state couple growth at some specific velocity. The mechanism of coupled growth in this case was shown to be operative due to the presence of a finite contact angle at the wall, and this was demonstrated by including the contact angle effect at the wall in the rod eutectic growth model. The experimental results were summarized to map out the conditions of thermal gradient and velocity on the regimes of composite and oscillatory microstructure formation. The formation of complex time-dependent microstructures was then discussed in terms of the time-dependent dynamics of planar interface growth.

Shin, Jong Ho

43

Role of Nucleation and Growth in Two-Phase Microstructure Formation  

SciTech Connect

During the directional solidification of peritectic alloys, a rich variety of two-phase microstructures develop, and the selection process of a specific microstructure is complicated due to the following two considerations. (1) In contrast to many single phase and eutectic microstructures that grow under steady state conditions, two-phase microstructures in a peritectic system often evolve under non-steady-state conditions that can lead to oscillatory microstructures, and (2) the microstructure is often governed by both the nucleation and the competitive growth of the two phases in which repeated nucleation can occur due to the change in the local conditions during growth. In this research, experimental studies in the Sn-Cd system were designed to isolate the effects of nucleation and competitive growth on the dynamics of complex microstructure formation. Experiments were carried out in capillary samples to obtain diffusive growth conditions so that the results can be analyzed quantitatively. At high thermal gradient and low velocity, oscillatory microstructures were observed in which repeated nucleation of the two phases was observed at the wall-solid-liquid junction. Quantitative measurements of nucleation undercooling were obtained for both the primary and the peritectic phase nucleation, and three different ampoule materials were used to examine the effect of different contact angles at the wall on nucleation undercooling. Nucleation undercooling for each phase was found to be very small, and the experimental undercooling values were orders of magnitude smaller than that predicted by the classical theory of nucleation. A new nucleation mechanism is proposed in which the clusters of atoms at the wall ahead of the interface can become a critical nucleus when the cluster encounters the triple junction. Once the nucleation of a new phase occurs, the microstructure is found to be controlled by the relative growth of the two phases that give rise to different oscillatory microstructures that depend on the imposed velocity and the size of the sample. At low thermal gradient to velocity ratio, a steady-state composite microstructure is observed. Two mechanisms of composite microstructure formation were examined: (1) the formation of the peritectic phase in the intercellular region of the primary phase where the solute rejected by the primary phase is absorbed by the peritectic phase. The peritectic phase forms a small distance behind the growing primary phase front. (2) The second mechanism is the coupled growth of the two phases with a macroscopically planar interface, as in the case of eutectic growth. Detailed studies showed that this composite microstructure, although it appears as a eutectic microstructure, did not grow in the coupled manner at the advancing interface in the Sn-cd system. However, a new observation was made when experiments were carried out in thin ampoule of Ta. The peritectic phase nucleated at the wall-interface triple junction and grew along the wall, while the primary phase continued to grow at the center, giving rise to a steady-state couple growth at some specific velocity. The mechanism of coupled growth in this case was shown to be operative due to the presence of a finite contact angle at the wall, and this was demonstrated by including the contact angle effect at the wall in the rod eutectic growth model. The experimental results were summarized to map out the conditions of thermal gradient and velocity on the regimes of composite and oscillatory microstructure formation. The formation of complex time-dependent microstructures was then discussed in terms of the time-dependent dynamics of planar interface growth.

Jong Ho Shin

2008-05-01

44

Interplay between ?(Ti) nucleation and growth during peritectic solidification investigated by phase-field simulations.  

PubMed

The properties of modern TiAl-based alloys with aluminum contents around 45 at.% critically depend on the as solidified ?(Ti) grain structure. Commonly, a rather coarse grain structure is obtained if ?(Ti) forms via the peritectic reaction '[Formula: see text]'. Phase-field simulations have been applied to perform a case study of grain structure formation during the early peritectic growth under unidirectional growth conditions. In the absence of foreign nucleation sites, the peritectic ?(Ti) phase nucleates on the dendritic surface of the properitectic ?(Ti) phase. For typical values of the critical nucleation undercooling, coarse structures with large elongated grains are reproduced. A delicate interplay between nucleation and growth is predicted for reduced values of the critical undercooling. In this case, the alloy composition is found to play an additional role. An effective grain refinement by frequent nucleation is obtained, if potent nucleants can reduce the critical undercooling below the local growth undercooling. Complementary Scheil calculations and Bridgman experiments show that in situ precipitation of TiB(2) particles can be controlled by adequate boron addition. Both, numerical predictions and experiments confirm that these particles can act as effective nucleation agents and significantly reduce the grain size of ?(Ti). PMID:21715868

Eiken, J; Apel, M; Witusiewicz, V T; Zollinger, J; Hecht, U

2009-10-27

45

Study of gypsum crystal nucleation and growth rates in simulated flue gas desulfurization liquors. Final report  

SciTech Connect

The kinetics of gypsum crystal nucleation and growth rates were measured in flue gas desulfurization (FGD) scrubber liquors. Variables studied were parent seed crystal size and mass; the organic additives citric acid, adipic acid, sodium dodecylbenzene sulfonate, and Calgon CL246 polyacrylic acid formulation; and pH. Citric acid produced gypsum crystals with a more favorable columnar structure. Lower pH resulted in increased nucleation rates. Stable secondary nucleation was observed in the presence of retained parent gypsum seed crystals of size >150 ..mu..m. Growth and nucleation rates were correlated using reaction kinetic models. These kinetics were then used in rigorous computer simulations to predict crystal-size distribution (CSD) with different scrubber configurations. Scrubber process configurations employing classified product removal were calculated to produce a gypsum sludge having a mean particle size up to twice as large as the particle size with unclassified operation.

Randolph, A.D.; Etherton, D.

1981-06-01

46

Theoretical study of vapor-liquid homogeneous nucleation using stability analysis of a macroscopic phase.  

PubMed

Stability analysis is generally used to verify that the solution to phase equilibrium calculations corresponds to a stable state (minimum of the free energy). In this work, tangent plane distance analysis for stability of macroscopic mixtures is also used for analyzing the nucleation process, reconciling thus this analysis with classical nucleation theories. In the context of the revised nucleation theory, the driving force and the nucleation work are expressed as a function of the Lagrange multiplier corresponding to the mole fraction constraint from the minimization problem of stability analysis. Using a van der Waals fluid applied to a ternary mixture, Lagrange multiplier properties are illustrated. In particular, it is shown how the Lagrange multiplier value is equal to one on the binodal and spinodal curves at the same time as the driving force of nucleation vanishes on these curves. Finally, it is shown that, on the spinodal curve, the nucleation work from the revised and generalized nucleation theories are characterized by two different local minima from stability analysis, irrespective of any interfacial tension models. PMID:23061836

Carreón-Calderón, Bernardo

2012-10-14

47

Control of protein crystal nucleation around the metastable liquid-liquid phase boundary.  

PubMed

The capability to enhance or suppress the nucleation of protein crystals opens opportunities in various fundamental and applied areas, including protein crystallography, production of protein crystalline pharmaceuticals, protein separation, and treatment of protein condensation diseases. Herein, we show that the rate of homogeneous nucleation of lysozyme crystals passes through a maximum in the vicinity of the liquid-liquid phase boundary hidden below the liquidus (solubility) line in the phase diagram of the protein solution. We found that glycerol and polyethylene glycol (which do not specifically bind to proteins) shift this phase boundary and significantly suppress or enhance the crystal nucleation rates, although no simple correlation exists between the action of polyethylene glycol on the phase diagram and the nucleation kinetics. The control mechanism does not require changes in the protein concentration, acidity, and ionicity of the solution. The effects of the two additives on the phase diagram strongly depend on their concentration, which provides opportunities for further tuning of nucleation rates. PMID:10823898

Galkin, O; Vekilov, P G

2000-06-01

48

Gas–vapor bubble nucleation—a unified approach  

Microsoft Academic Search

In a solution which is saturated with gas near the superheat limit, one might expect a bubble formed from both dissolved gas and vapor molecules to appear. The integration of the surface-energy concepts, that are postulated on completely different physical bases for gas and vapor bubble formation is a major issue. In this paper, we reformulate gas and vapor bubble

Ho-Young Kwak; Si-Doek Oh

2004-01-01

49

Nucleation, growth and ageing scenarios in closed systems II: Dynamics of a new phase formation  

Microsoft Academic Search

We simulate the dynamics of nucleation, growth and resorption of particles in an initially supersaturated closed medium, according to the approach worked out in the preceding paper, which applies to precipitation in solution, condensation in the vapor phase or crystallization from a melt. We find that the formation of the new phase obeys a generic scenario, involving mainly three stages,

C. Noguera; B. Fritz; A. Clément; A. Baronnet

2006-01-01

50

Helmholtz free energy of a phase containing a sparse ensemble of heterophase clusters with application to nucleation theory.  

PubMed

A decomposition of the Helmholtz free energy of a phase containing a sparse ensemble of heterophase clusters is derived based on classical statistical mechanics and on the general physical characteristics of such systems. It is not assumed that the phase is an ideal gas. The building blocks of this decomposition are the Helmholtz free energies of the constituents (phase and stationary heterophase clusters) and, for every cluster species, a volume V(k)(cm), which is of the magnitude of the thermal fluctuation volume of the center of mass of the stationary cluster containing k monomers. A definition of V(k)(cm) is given in terms of the configuration integrals of the clusters. V(k)(cm) is evaluated for k > 1, with the result that V(k)(cm) is proportional to k(-1/2) and is a function of temperature, the specific volume, and the isothermal compressibility of the phase in the cluster. A thermodynamically consistent expression for the work to form a stationary cluster, which reads as Delta g(k)/(k(B)T) = -ak + (3/2)bk(2/3) + 3ck(1/3) + d, is derived. The coefficients a, b, c, and d depend on the thermodynamic properties of the homogeneous phases, on the surface tension, and on one additional phenomenological material function of temperature and pressure. The description is general and covers a wide class of materials. It is shown that the heterogeneous system represents the thermodynamic equilibrium and not the pure phase without clusters. The resulting expression for the equilibrium particle number, which is different from the one used in classical nucleation theory, is by a standard procedure input for the calculation of the stationary Becker-Döring nucleation rate and entails a correction factor for the classical nucleation rate. Comparison with experiments is provided for nucleation onset measurements of argon and for measurements of the homogeneous nucleation rate of water. Measurements and theory can be brought to match within the limits of experimental precision in both cases. PMID:20148535

Tschudi, Hans Rudolf

2010-03-11

51

Nucleation, growth, and coarsening for two- and three-dimensional phase transitions  

NASA Astrophysics Data System (ADS)

Phase transitions from a metastable state often occur by nucleation accompanied by particle growth and eventually by Ostwald coarsening. In closed systems, the supersaturation declines as particles nucleate and grow, causing an increase in the stable critical nucleus size. Particles below the critical size dissolve spontaneously during coarsening and their mass is released to contribute to further growth of remaining particles. By developing a population balance model that represents nucleation, growth, and coarsening, we here determine the dynamics of particle size distributions (PSDs). The governing equations are solved numerically to show that the transition from nucleation and growth to coarsening occurs over a relatively long time period. The asymptotic coarsening stage reveals a power-law increase in average particle mass as the PSD evolves to a (minimum) polydispersity index of unity for both two-dimensional (2-D) and 3-D phase transitions. The model agrees with published conclusions that nucleation and coarsening overlap when interfacial energy is small or supersaturation is large.

Madras, Giridhar; McCoy, Benjamin J.

2005-06-01

52

Aerosol formation by gas-phase-ozonolysis of small vinylethers  

NASA Astrophysics Data System (ADS)

Secondary organic aerosol (SOA) is formed in the troposphere by ozonolysis of biogenic VOC. The gas-phase processes however, which follow the ozonolysis reaction and lead to the formation of the nucleating compounds are so far poorly understood. A wide range of different primary or secondary products of gas-phase ozonolyses has so far been suggested as potential precursor of nucleation, such as secondary ozonides, dicarboxylic acids and their dimers, and peroxy compounds, such as peroxyhemiacetales or diacylperoxides. In order to assess the influences of oxygen groups in the neighborhood of the alkene double bond on reaction products and particle formation, gas-phase ozonolysis reactions of small vinylethers are investigated in this work. The observations might allow to draw conclusion on the likelihood of potential particle precursors to be formed in the gas phase and to have a sufficiently low vapour pressure for nucleation. Experiments have been carried out in a flow reactor of 1 m length in order to observe SOA formation during the first minute of the reaction, and in a 570 l spherical glass reactor in order to study nucleation or product formation dependent on the reaction time longer than 1 min. Particle size distributions are measured with a scanning mobility particle analyser (SMPS, Model 3936, TSI Inc.), product formation in the gas-phase is analysed by FTIR, and in the particle phase by LC-MS-MS. Threshold alkene conversions necessary for nucleation and initial particle formation are determined in the flow reactor by varying alkene concentrations at constant ozone concentrations and reaction times. Moreover, the influence of experimental conditions, such as relative humidity, is studied.

Sadezky, A.; Mellouki, A.; Winterhalter, R.; Römpp, A.; Moortgat, G.

2003-04-01

53

Investigation of nucleation and propagation of phase transitions in TiNi SMA  

Microsoft Academic Search

The attention of the present paper is focused on the aid provided by infrared thermography, for spectacular investigation of nucleation and further development of the stress induced phase transitions in TiNi shape memory alloy. This is a qualitative analysis aimed to verify the feasibility of further study in the application of IR for studying change phenomena. To this end, the

El?bieta Pieczyska; Stefan Gadaj; Wojciech Nowacki; Hisaaki Tobushi

2004-01-01

54

Random-Walk Monte Carlo Simulation of Intergranular Gas Bubble Nucleation in UO2 Fuel  

SciTech Connect

Using a random-walk particle algorithm, we investigate the clustering of fission gas atoms on grain bound- aries in oxide fuels. The computational algorithm implemented in this work considers a planar surface representing a grain boundary on which particles appear at a rate dictated by the Booth flux, migrate two dimensionally according to their grain boundary diffusivity, and coalesce by random encounters. Specifically, the intergranular bubble nucleation density is the key variable we investigate using a parametric study in which the temperature, grain boundary gas diffusivity, and grain boundary segregation energy are varied. The results reveal that the grain boundary bubble nucleation density can vary widely due to these three parameters, which may be an important factor in the observed variability in intergranular bubble percolation among grain boundaries in oxide fuel during fission gas release.

Yongfeng Zhang; Michael R. Tonks; S. B. Biner; D.A. Andersson

2012-11-01

55

The global influence of dust mineralogical composition on heterogeneous ice nucleation in mixed-phase clouds  

NASA Astrophysics Data System (ADS)

Mineral dust is the dominant natural ice nucleating aerosol. Its ice nucleation efficiency depends on the mineralogical composition. We show the first sensitivity studies with a global climate model and a three-dimensional dust mineralogy. Results show that, depending on the dust mineralogical composition, coating with soluble material from anthropogenic sources can lead to quasi-deactivation of natural dust ice nuclei. This effect counteracts the increased cloud glaciation by anthropogenic black carbon particles. The resulting aerosol indirect effect through the glaciation of mixed-phase clouds by black carbon particles is small (+0.1 W m-2 in the shortwave top-of-the-atmosphere radiation in the northern hemisphere).

Hoose, C.; Lohmann, U.; Erdin, R.; Tegen, I.

2008-04-01

56

Nucleation of a non-critical phase in a fluid near a critical point  

Microsoft Academic Search

Phase diagrams of some globular proteins have a fluid-fluid transition as\\u000awell as a fluid-crystal transition. Homogeneous nucleation of the crystal from\\u000athe fluid phase near the critical point of the fluid-fluid transition is\\u000aexamined. As the fluid-fluid critical point is approached, the number of\\u000amolecules in the critical nucleus, the nucleus at the top of the free energy\\u000abarrier

Richard P. Sear

2000-01-01

57

Nucleation in finite topological systems during continuous metastable quantum phase transitions.  

PubMed

Finite topological quantum systems can undergo continuous metastable quantum phase transitions to change their topological nature. Here we show how to nucleate the transition between ring currents and dark soliton states in a toroidally trapped Bose-Einstein condensate. An adiabatic passage to wind and unwind its phase is achieved by explicit global breaking of the rotational symmetry. This could be realized with current experimental technology. PMID:23004576

Fialko, Oleksandr; Delattre, Marie-Coralie; Brand, Joachim; Kolovsky, Andrey R

2012-06-20

58

Classical nucleation theory with a radius-dependent surface tension: A two-dimensional lattice-gas automata model  

NASA Astrophysics Data System (ADS)

The constant surface tension assumption of the Classical Nucleation Theory (CNT) is known to be flawed. In order to probe beyond this limitation, we consider a microscopic, two-dimensional Lattice-Gas Automata (LGA) model of nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water bonding energy), ? (next-to-nearest-neighbor bonding coefficient in solid phase), and Cin (initial solute concentration). The LGA method has the advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system's concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the equilibrium concentration. The “Mean First-Passage Time” technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation data are evaluated using a modification to the CNT that incorporates a two-dimensional radius-dependent surface tension term. The Tolman parameter, ?, which controls the radius dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of ? and Esw (Ess). On the other hand, ? increases as ? increases while Ess and Esw are held constant. The constant surface tension term of the CNT, ?0, increases (decreases) with increasing magnitudes of Ess (Esw) at fixed values of Esw (Ess) and increases as ? is increased. ?0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |?E|, however, with a slope less than that predicted for a crystal that is uniformly packed at maximum density. These results indicate an increase in the radius-dependent surface tension, ?, with respect to increasing magnitude of the difference between Ess and Esw.

Hickey, Joseph; L'Heureux, Ivan

2013-02-01

59

A numerical method for the study of nucleation of ordered phases  

NASA Astrophysics Data System (ADS)

A numerical approach based on the string method is developed to study nucleation of ordered phases in first-order phase transitions. Among other things, this method allows an efficient computation of the minimum energy path (MEP) during the nucleation process. The MEP provides information about the size, shape and free energy barrier of the critical nucleus. To improve the efficiency of the string method, a special initialization process is proposed. Constraints from physical models are treated using two methods, a generalized coordinates method and a projection method. Strategies for choosing the computational domain and defining the nucleus boundary are also introduced. The validity of our approach is illustrated by two nontrivial examples from soft condensed matter physics, namely the nematic-isotropic transition of liquid crystals and the ordered-to-ordered phase transition of diblock copolymers.

Lin, Ling; Cheng, Xiuyuan; E, Weinan; Shi, An-Chang; Zhang, Pingwen

2010-03-01

60

Quantized hard-x-ray phase vortices nucleated by aberrated nanolenses  

SciTech Connect

Quantized x-ray phase vortices, namely, screw-type topological defects in the wave fronts of a coherent monochromatic scalar x-ray wave field, may be spontaneously nucleated by x-ray lenses. Phase retrieval is used to reconstruct the phase and amplitude of the complex disturbance created by aberrated gold nanolenses illuminated with hard x rays. A nanoscale quantized x-ray vortex-antivortex dipole is observed, manifest both as a pair of opposite-helicity branch points in the Riemann sheets of the multivalued x-ray phase map of the complex x-ray field and in the vorticity of the associated Poynting vector field.

Pavlov, Konstantin M. [School of Science and Technology, University of New England, Armidale, New South Wales 2351 (Australia); School of Physics, Monash University, Victoria 3800 (Australia); Paganin, David M. [School of Physics, Monash University, Victoria 3800 (Australia); Vine, David J. [ARC Centre of Excellence for Coherent X-ray Science, School of Physics, The University of Melbourne, Parkville, Victoria 3010 (Australia); Schmalz, Jelena A. [School of Science and Technology, University of New England, Armidale, New South Wales 2351 (Australia); Suzuki, Yoshio; Uesugi, Kentaro; Takeuchi, Akihisa; Yagi, Naoto [SPring-8/JASRI (Japan Synchrotron Radiation Research Institute), Hyogo 679-5198 (Japan); Kharchenko, Alexander; Blaj, Gabriel [PANalytical B.V., P.O. Box 13, 7600 AA Almelo (Netherlands); Jakubek, Jan [Institute of Experimental and Applied Physics, Czech Technical University in Prague, 166 36 Prague 6 (Czech Republic); Altissimo, Matteo [Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, Victoria 3168 (Australia); Materials Science and Engineering, Commonwealth Scientific and Industrial Research Organisation, Clayton South, Victoria 3169 (Australia); Clark, Jesse N. [London Centre for Nanotechnology, University College, Gower St, London WC1E 6BT (United Kingdom)

2011-01-15

61

Study of gypsum crystal nucleation and growth rates in simulated flue gas desulfurization liquors. Final report  

Microsoft Academic Search

The kinetics of gypsum crystal nucleation and growth rates were measured in flue gas desulfurization (FGD) scrubber liquors. Variables studied were parent seed crystal size and mass; the organic additives citric acid, adipic acid, sodium dodecylbenzene sulfonate, and Calgon CL246 polyacrylic acid formulation; and pH. Citric acid produced gypsum crystals with a more favorable columnar structure. Lower pH resulted in

A. D. Randolph; D. Etherton

1981-01-01

62

Study on the ? to ? transformation of PP\\/POE blends with ?-phase nucleating agent during the tensile deformation process  

Microsoft Academic Search

As a part of serial work about the toughening effect of elastomer and nucleating agent on polypropylene (PP), this work is focused on the microstructure changes of PP matrix in PP\\/elastomer blends with ?-phase nucleating agent (?-NA) during the uniaxial tensile deformation process. The microstructure changes have been investigated through differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD)

Xiaoxi Li; Haiyan Wu; Yong Wang; Hongwei Bai; Li Liu; Ting Huang

2010-01-01

63

First-order chiral phase transition in high-energy collisions: Can nucleation prevent spinodal decomposition?  

SciTech Connect

We discuss homogeneous nucleation in a first-order chiral phase transition within an effective field theory approach to low-energy QCD. Exact decay rates and bubble profiles are obtained numerically and compared to analytic results obtained with the thin-wall approximation. The thin-wall approximation overestimates the nucleation rate for any degree of supercooling. The time scale for critical thermal fluctuations is calculated and compared to typical expansion times for high-energy hadronic or heavy-ion collisions. We find that significant supercooling is possible, and the relevant mechanism for phase conversion might be that of spinodal decomposition. Some potential experimental signatures of supercooling, such as an increase in the correlation length of the scalar condensate, are also discussed.

Scavenius, O.; Dumitru, A.; Fraga, E. S.; Lenaghan, J. T.; Jackson, A. D.

2001-06-01

64

Large-grained polycrystalline Si films obtained by selective nucleation and solid phase epitaxy  

Microsoft Academic Search

We investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin film crystalline silicon solar cells. Since the use of glass substrates constrains process temperatures, our chosen approach to form large-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selective crystallization of an initially amorphous silicon film, at

R. A Puglisi; H Tanabe; C. M Chen; Harry A Atwater

2000-01-01

65

Nucleation and phase propagation in a multistable lattice with weak nonlocal interactions  

Microsoft Academic Search

We study the overdamped gradient flow dynamics of a chain of massless points connected by bistable nearest-neighbor (NN) interactions\\u000a and harmonic next-nearest-neighbor (NNN) interactions under quasistatic loads of assigned displacements. The model reproduces\\u000a experimental observations on the phase transition of shape-memory wires with the possibility of different microstructure evolution\\u000a strategies: internal or boundary nucleations and one or two coherently propagating

G. Puglisi

2007-01-01

66

Phase-field modelling for metals and colloids and nucleation therein—an overview  

NASA Astrophysics Data System (ADS)

Phase-field modelling, as it is understood today, is still a young discipline in condensed matter physics, which established itself for that class of systems in condensed matter physics, which can be characterized by domains of different phases separated by a distinct interface. Driven out of equilibrium, their dynamics results in the evolution of those interfaces, during which those might develop into well-defined structures with characteristic length scales at the nano-, micro- or mesoscale. Since the material properties of such systems are, to a large extent, determined by those small-scale structures, acquiring a precise understanding of the mechanisms that drive the interfacial dynamics is a great challenge for scientists in this field. Phase-field modelling is an approach that allows us to tackle this challenge simulation-based. This overview summarizes briefly the essentials of the conceptual background of the phase-field method, as well as recent issues the phase-field community is focusing on, as far as they are related to nucleation. To that end a brief introduction to the basic understanding underlying the diffuse interface description, which is the conceptual backbone of phase-field modelling, is given at the beginning, followed by a detailed picture of its achievements so far in applications to nucleation phenomena in metals and colloids. Within the most relevant fields of condensed matter physics, approached by phase-field modelling until now, applications to metallic systems are a traditional domain of phase-field modelling and nucleation phenomena therein have been addressed by several groups. This paper provides an overview of these. Advances in the field of colloidal systems, on the other hand, are only more recent and are addressed here in the context of contributions to soft matter physics in general.

Emmerich, H.

2009-11-01

67

Nucleation and growth of the Alpha-Prime Phase martensitic phase in Pu-Ga Alloys  

SciTech Connect

In a Pu-2.0 at% Ga alloy, it is observed experimentally that the amount of the martensitic alpha-prime product formed upon cooling the metastable delta phase below the martensite burst temperature (M{sub b}) is a function of the holding temperature and holding time of a prior conditioning (''annealing'') treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the alpha-prime phase, it was first homogenized for 8 hours at 375 C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between -50 C and 370 C for periods of up to 70 hours to determine the holding time and temperature that produced the largest volume fraction of alpha-prime upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of alpha-prime formed, the largest amount of alpha-prime was obtained following holding at 25 C for at prime least 6 hours. Additional time at 25 C, up to 70 hours, did not increase the amount of subsequent alpha-prime formation. At 25 C, the Pu-2.0 at% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are {alpha} and Pu{sub 3}Ga, although a complete eutectoid decomposition of delta to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of alpha-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic alpha-prime particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classical nucleation theory is presented to describe the formation of alpha-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more alpha-prime formation on cooling are discussed in terms of the differences in the potency of the embryos created in the delta matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the alpha-prime formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing.

Blobaum, K M; Krenn, C R; Wall, M A; Massalski, T B; Schwartz, A J

2005-02-09

68

Quantification of gypsum crystal nucleation, growth, and breakage rates in a wet flue gas desulfurization pilot plant  

Microsoft Academic Search

The aim of this work is to study the influence of nucleation, growth and breakage on the particle size distribution (PSD) of gypsum crystals produced by the wet flue gas desulfurization (FGD) process. The steady state PSD, obtained in a falling film wet FGD pilot plant during desulfurization of a 1000 ppm(V) SO gas stream, displayed a strong nonlinear behaviour

Brian B. Hansen; Søren Kiil; Jan E. Johnsson

2009-01-01

69

Heterogeneous nucleation and growth of the ?(Ti) phase in the Ti-Al system-experiments and model calculations.  

PubMed

The barrier to heterogeneous nucleation of the ?(Ti) phase on TiB(2) and other borides has been evaluated using the plane to plane matching model. The results are compared to the known nucleation of the ?(Ti) phase on the ?(Ti) phase. According to this comparison, the barrier to heterogeneous nucleation of the ?(Ti) phase on TiB(2) can be judged to be small. This is in agreement with inoculation experiments. The addition of a Ti-Al-TiB(2) master composite to a ?(Ti) solidifying TiAl based alloy leads to a significantly refined microstructure. Microsegregations enable us to attribute this refinement to refined equiaxed ?(Ti) dendrites. However, model calculations based on the hemispherical cap model predict that the refinement via heterogeneous ?(Ti) nucleation should be more potent. First calculations indicate that structural imperfections of TiB(2) particles limit the nucleation site diameter. Thereby, the nucleation barrier is increased and the refinement is less pronounced. PMID:21715875

Gosslar, Daniel; Hartig, Christian; Günther, Robert; Hecht, Ulrike; Bormann, Rüdiger

2009-10-27

70

Aerosol Inflluence on Ice Nucleation via the Immersion Mode in Mixed-Phase Arctic Stratiform Clouds  

NASA Astrophysics Data System (ADS)

Mixed-phase stratiform clouds are commonly observed at high latitudes (e.g. Shupe et al., 2006; de Boer et al., 2009a). Herman and Goody (1976), as well as Curry et al. (1996) present summaries of Arctic cloud climatologies that show low altitude stratus frequencies of up to 70% during transitional seasons. In addition to their frequent occurrence, these clouds have significant impacts on the near-surface atmospheric radiative budget, with estimates of wintertime reductions in net surface cooling of 40-50 Wm-2 (Curry et al., 1996) due predominantly to liquid in the mixed-phase layer. Both observational and modeling studies (e.g. Harrington et al., 1999; Jiang et al., 2000; Shupe et al., 2008; Klein et al., 2008) show a strong connection between the amount of ice present and the lifetime of the liquid portion of the cloud layer. This is thought to occur via the Bergeron-Findeissen mechanism (Pruppacher and Klett, 1997) in which ice grows at the expense of liquid due to its lower saturation vapor pressure. Unfortunately, the mechanisms by which ice is nucleated within these mixed-phase layers are not yet fully understood, and therefore an accurate depiction of this process for mixed-phase stratiform clouds has not yet been characterized. The nucleation mechanisms that are active in a given environment are sensitive to aerosol properties. Insoluble particles are typically good nuclei for ice particle formation, while soluble particles are typically better at nucleating water droplets. Aerosol observations from the Arctic often show mixed aerosol particles that feature both soluble and insoluble mass (Leaitch et al., 1984). Soluble mass fractions for these particles have been shown to be high, with estimates of 60-80% and are often made up of sulfates (Zhou et al., 2001; Bigg and Leck, 2001). It is believed that a significant portion of this sulfate mass comes from dimethyl sulfide (DMS) production in the Arctic Ocean and subsequent atmospheric oxidation. Since these mixed particles may initially nucleate liquid droplets that then contain the remaining insoluble mass, immersion freezing has been theorized to contribute to ice nucleation in these clouds (de Boer et al., 2009b). In this work, we present a numerical study focusing on immersion freezing in mixed-phase stratiform clouds. We investigate the effects of aerosol properties such as soluble mass fraction, insoluble mass type and aerosol concentration on cloud liquid water fraction and lifetime. The high-resolution simulations used are based on the current GEWEX Cloud System Study (GCSS)/World Meteorological Organization (WMO) mixed-phase stratus model intercomparison (Morrison and Zuidema, 2008) and are completed using the University of Wisconsin Non-Hydrostatic Modeling System (UW-NMS, Tripoli, 1992) in combination with an advanced bin microphysical scheme (AMPS, Hashino and Tripoli, 2008). Immersion freezing is represented using a parameterization from Diehl and Wurzler (2004), which accounts for the freezing point depression due to the concentration of soluble mass within the water droplet.

de Boer, G.; Hashino, T.; Tripoli, G. J.; Eloranta, E. W.

2009-12-01

71

Homogeneous Crystal Nucleation Near a Metastable Fluid-Fluid Phase Transition  

NASA Astrophysics Data System (ADS)

Several scenarios exist for the protein crystallization and aggregation in solutions near a metastable fluid-fluid phase separation below the solubility line. Based on computations, it was proposed that the fluid-fluid critical point enhances the crystallization rate by many orders of magnitude, while, based on experiments, it was proposed that the fluid-fluid spinodal controls the crystallization rate. Using molecular dynamic simulations for an isotropic model with sticky interaction, we show that neither of these scenarios adequately describes the crystallization mechanism near a metastable fluid-fluid phase separation. We find that the emergence of the high-density fluid inside the spinodal drastically enhances the crystal nucleation in the subcritical region following Ostwald’s rule of stages.

Xu, Limei; Buldyrev, Sergey V.; Stanley, H. Eugene; Franzese, Giancarlo

2012-08-01

72

Inherent structures of phase-separating binary mixtures: Nucleation, spinodal decomposition, and pattern formation  

NASA Astrophysics Data System (ADS)

An energy landscape view of phase separation and nonideality in binary mixtures is developed by exploring their potential energy landscape (PEL) as functions of temperature and composition. We employ molecular dynamics simulations to study a model that promotes structure breaking in the solute-solvent parent binary liquid, at low temperatures. The PEL of the system captures the potential energy distribution of the inherent structures (IS) of the system and is obtained by removing the kinetic energy (including that of intermolecular vibrations). The broader distribution of the inherent structure energy for structure breaking liquid than that of the structure making liquid demonstrates the larger role of entropy in stabilizing the parent liquid of the structure breaking type of binary mixtures. At high temperature, although the parent structure of the structure breaking binary mixture is homogenous, the corresponding inherent structure is found to be always phase separated, with a density pattern that exhibits marked correlation with the energy of its inherent structure. Over a broad range of intermediate inherent structure energy, bicontinuous phase separation prevails with interpenetrating stripes as signatures of spinodal decomposition. At low inherent structure energy, the structure is largely phase separated with one interface where as at high inherent structure energy we find nucleation type growth. Interestingly, at low temperature, the average inherent structure energy () exhibits a drop with temperature which signals the onset of crystallization in one of the phases while the other remains in the liquid state. The nonideal composition dependence of viscosity is anticorrelated with average inherent structure energy.

Sarkar, Sarmistha; Bagchi, Biman

2011-03-01

73

Thermodynamics, gas-liquid nucleation, and size-dependent surface tension  

NASA Astrophysics Data System (ADS)

Phenomenological nucleation theories are considered from the viewpoint of Gibbs' surface thermodynamics. We point out, in defining the critical nucleus, that it is important to make a distinction between the number of molecules enclosed by the surface of tension and the excess number of molecules over the uniform vapor phase. We show that the Kelvin equation should be employed in determining the size of the critical nucleus even if the nucleus free energy contains a size-dependent surface energy term. Furthermore, we make use of the fact that the classical form of Kelvin equation (containing the surface tension of a flat interface) predicts the equimolar radius of the critical nucleus well down to nuclei of about 40 molecules, and derive a new equation for the size-dependent surface tension that differs from the Tolman relation. Density functional calculations support the new formula.

Laaksonen, A.; McGraw, R.

1996-08-01

74

Electroweak phase transition nucleation with the MSSM and electromagnetic field creation  

SciTech Connect

Using EW-MSSM field theory, so the electroweak phase transition (EWPT) is first order, we derive the equations of motion (e.o.m.) for the gauge fields. With an isospin ansatz we derive e.o.m. for the electrically charged W fields uncoupled from all other fields. These and the lepton currents serve as the current for the Maxwell-like e.o.m. for the electromagnetic field. The electromagnetic field arising during EWPT bubble nucleation without leptons is found. We then calculate the electron current contribution, which is seen to be quite large. This provides the basis for determining the magnetic field created by EWPT bubble collisions, which could seed galactic and extragalactic magnetic fields.

Henley, Ernest M.; Johnson, Mikkel B.; Kisslinger, Leonard S. [Department of Physics, University of Washington, Seattle, Washington 98195 (United States); Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 (United States)

2010-04-15

75

An anisotropic phase-field crystal model for heterogeneous nucleation of ellipsoidal colloids.  

PubMed

We derive a generalized model for isotropic as well as anisotropic crystal lattice systems of arbitrary Poisson ratio within the framework of the continuum phase-field crystal (PFC) approach (Elder and Grant 2004 Phys. Rev. E 70 051606). To this end we extend the simplest PFC model defined by a free energy functional, which is based upon the Swift-Hohenberg model of pattern formation (Swift and Hohenberg 1993 Phys. Rev. A 15 851) to a conservative, anisotropic Langevin equation. By studying heterogeneous nucleation of ellipsoidal colloids at a wall, we demonstrate the capacity of our approach to contribute to the more precise understanding of condensed matter systems built up from non-spherical units at the atomic scale. In particular we address the question of how (a) the orientation of the ellipsoids as well as (b) the interaction potential with the wall determine the resulting contact angle. PMID:21715874

Prieler, R; Hubert, J; Li, D; Verleye, B; Haberkern, R; Emmerich, H

2009-10-27

76

A model for calculation of isothermal kinetics of the nucleation-and-growth type phase separation in the course of one-step heat treatment  

Microsoft Academic Search

A model is described for calculating the kinetics of a diffusion-limited nucleation-and-growth type phase separation. A homogeneous melt of glass is cooled to a temperature, at which the nucleation, growth and Ostwald ripening of particles of a new phase are possible. In the model, a spatial inhomogeneity of nucleation rate, caused by the formation of diffusion zones around particles, and

M. P. Shepilov

1996-01-01

77

Molecular dynamics of simulation of the nucleation, growth, inhibition, and control of gas hydrates. Annual report, April 1991March 1992  

Microsoft Academic Search

This is the first annual report for the Gas Research Institute-funded contract (number 5091-860-2125) entitled 'Molecular Dynamics of Gas Hydrate Nucleation, Growth, Inhibition and Control', covering the period 4\\/1\\/91-4\\/1\\/92. There were two main objectives of the research performed in 1991\\/1992: First, it was important to characterize the thermodynamic, structural and kinetic behavior of a computer-simulated hydrate to ensure that the

1992-01-01

78

Gas-phase polar cycloadditions  

NASA Astrophysics Data System (ADS)

The dilute gas-phase environment of mass spectrometers provides versatile and convenient medium in which chemists can form a great variety of ionic species and study their intrinsic reactivity exploring reaction mechanisms and screening for new reactions and their applications. Recent and illustrative examples of gas-phase polar cycloadditions of even and odd-electrons ions, which have been observed (some for the first time) in the solvent- and counter ion-free environment of mass spectrometers, are presented. Synthetic and analytical applications of such reactions and correlations with analogous reactions observed in solution are also discussed.

Eberlin, Marcos N.

2004-07-01

79

Kinetics of solid hydrate formation by carbon dioxide: Phase field theory of hydrate nucleation and magnetic resonance imagingy  

Microsoft Academic Search

In the course of developing a general kinetic model of hydrate formation\\/reaction that can be used to establish\\/ optimize technologies for the exploitation of hydrate reservoirs, two aspects of CO2 hydrate formation have been studied. (i) We developed a phase field theory for describing the nucleation of CO2 hydrate in aqueous solutions. The accuracy of the model has been demonstrated

B. Kvamme; A. Graue; T. Kuznetsova; L. Granasy; G. Toth; T. Pusztaib; G. Tegzeb

2004-01-01

80

Inclusion phases and the nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels  

Microsoft Academic Search

Series of submerged arc welds of HSLA steel made with three different fluxes and metallic additions of Ti, Mo, and Cr have been examined to study the inclusions and their role in the nucleation of acicular ferrite. Inclusion phases and compositions have been analyzed by electron diffraction and X-ray microanalysis. These analyses have shown that the inclusions contained many different

J. M. Dowling; J. M. Corbett; H. W. Kerr

1986-01-01

81

Inherent structures of phase-separating binary mixtures: nucleation, spinodal decomposition, and pattern formation.  

PubMed

An energy landscape view of phase separation and nonideality in binary mixtures is developed by exploring their potential energy landscape (PEL) as functions of temperature and composition. We employ molecular dynamics simulations to study a model that promotes structure breaking in the solute-solvent parent binary liquid, at low temperatures. The PEL of the system captures the potential energy distribution of the inherent structures (IS) of the system and is obtained by removing the kinetic energy (including that of intermolecular vibrations). The broader distribution of the inherent structure energy for structure breaking liquid than that of the structure making liquid demonstrates the larger role of entropy in stabilizing the parent liquid of the structure breaking type of binary mixtures. At high temperature, although the parent structure of the structure breaking binary mixture is homogenous, the corresponding inherent structure is found to be always phase separated, with a density pattern that exhibits marked correlation with the energy of its inherent structure. Over a broad range of intermediate inherent structure energy, bicontinuous phase separation prevails with interpenetrating stripes as signatures of spinodal decomposition. At low inherent structure energy, the structure is largely phase separated with one interface where as at high inherent structure energy we find nucleation type growth. Interestingly, at low temperature, the average inherent structure energy () exhibits a drop with temperature which signals the onset of crystallization in one of the phases while the other remains in the liquid state. The nonideal composition dependence of viscosity is anticorrelated with average inherent structure energy. PMID:21517506

Sarkar, Sarmistha; Bagchi, Biman

2011-03-28

82

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films  

NASA Astrophysics Data System (ADS)

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

Suh, J. Y.; Lopez, R.; Feldman, L. C.; Haglund, R. F.

2004-07-01

83

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films  

NASA Astrophysics Data System (ADS)

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

Suh, Jae Yong; Lopez, Rene; Feldman, Leonard C.; Haglund, Richard F.

2004-11-01

84

Formation of Large Chiral Domains in the B4 Phase by Nucleation and Chirality-Preserving Growth of Helical Filaments  

NASA Astrophysics Data System (ADS)

The growth of helical filaments in the B4 phase is investigated in mixtures of a bent-core mesogen (NOBOW) and a calamitic mesogen (8CB). Due to freezing point depression, the B4 phase forms directly from isotropic phase in the mixtures. This enables us to show, for the first time, that the left- and right- chiral domains are composed of left- and right- handed helical filaments respectively. The formation of a chiral domain can be described as a nucleation and growth process, starting from a nucleus of arbitrary chirality. Starting from such a nucleus, the local chirality is maintained by chirality-preserving growth of helical filaments. The formation of large chiral domains in the mixture is due to the low density of nucleation sites. A tree branching model of the B4 helical filament growth has been proposed which accounts for the observed local homochirality of the helical filaments and the local phase coherence between the helical filaments.

Chen, D.; Glaser, M. A.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M.

2010-03-01

85

PHASE-FIELD SIMULATION OF IRRADIATED METALS: PART II: GAS BUBBLE KINETICS  

SciTech Connect

We present a phase-field model for inert gas bubble formation and evolution in irradiated metals. The model evolves vacancy, self-interstitial, and fission gas atoms through a coupled set of Cahn-Hilliard and Allen-Cahn equations, capturing the processes of defect generation, recombination, annihilation at GB sinks, as well as intragranular and intergranular bubble nucleation and growth in polycrystalline microstructures. Illustrative results are presented that characterize bubble growth and shrinkage, as well as the bubble density, size and nucleation rate as a function of varying irradiation conditions. Finally, intergranular bubble characteristics such as shape, pinning energy on GB motion, and bubble density are investigated.

Paul C Millett; Anter El-Azab

2011-01-01

86

Nucleation stage with nonsteady growth of supercritical gas bubbles in a strongly supersaturated liquid solution and the effect of excluded volume  

Microsoft Academic Search

An approach to the kinetics of barrier formation of supercritical gas bubbles in a strongly supersaturated liquid solution is presented. A common assumption of uniform reduction of a dissolved gas supersaturation in a liquid solution via stationary diffusion to nucleating gas bubbles is shown to be not applicable to the case of high gas supersaturations. The approach recognizes that the

Anatoly E. Kuchma; Fedor M. Kuni; Alexander K. Shchekin

2009-01-01

87

Global model comparison of heterogeneous ice nucleation parameterizations in mixed phase clouds  

NASA Astrophysics Data System (ADS)

A new aerosol-dependent mixed phase cloud parameterization for deposition/condensation/immersion (DCI) ice nucleation and one for contact freezing are compared to the original formulations in a coupled general circulation model and aerosol transport model. The present-day cloud liquid and ice water fields and cloud radiative forcing are analyzed and compared to observations. The new DCI freezing parameterization changes the spatial distribution of the cloud water field. Significant changes are found in the cloud ice water fraction and in the middle cloud fractions. The new DCI freezing parameterization predicts less ice water path (IWP) than the original formulation, especially in the Southern Hemisphere. The smaller IWP leads to a less efficient Bergeron-Findeisen process resulting in a larger liquid water path, shortwave cloud forcing, and longwave cloud forcing. It is found that contact freezing parameterizations have a greater impact on the cloud water field and radiative forcing than the two DCI freezing parameterizations that we compared. The net solar flux at top of atmosphere and net longwave flux at the top of the atmosphere change by up to 8.73 and 3.52 W m-2, respectively, due to the use of different DCI and contact freezing parameterizations in mixed phase clouds. The total climate forcing from anthropogenic black carbon/organic matter in mixed phase clouds is estimated to be 0.16-0.93 W m-2using the aerosol-dependent parameterizations. A sensitivity test with contact ice nuclei concentration in the original parameterization fit to that recommended by Young (1974) gives results that are closer to the new contact freezing parameterization.

Yun, Yuxing; Penner, Joyce E.

2012-04-01

88

Order and phase nucleation in non-equilibrium nanocomposite Fe-Pt thin films with perpendicular magnetic anisotropy.  

SciTech Connect

We report on the time evolution of mass transport upon annealing nonequilibrium Fe-Pt nanocomposite films, leading to nucleation of L1{sub 0} chemically ordered phase. The nonequilibrium nanocomposite films were fabricated by applying Fe{sup +} ion implantation to epitaxial Pt films grown on (001) MgO substrates, yielding Fe nanoclusters embedded in a Pt matrix at a tailored penetration depth. Time-resolved x-ray diffraction studies were carried out using synchrotron radiation, allowing determination of the activation energy for nucleation of the FePt L1{sub 0} phase within the segregated nanoclusters during annealing. The growth of the segregated L1{sub 0} ordered phase was modeled using ideal grain-size law and found to be dominated by strain-driven surface nucleation. The activation energies were found to correlate with the nanocluster size. Magnetic characterization of selected annealed samples indicates perpendicular magnetic anisotropy with high coercive field coincident with high value of the chemical order parameter of the ordered phase within the magnetic nanoclusters.

Clavero, C.; Skuza, J. R.; Garcia-Martin, J. M.; Cebollada, A.; Walko, D. A.; Lukaszew, R. A.; Coll. of William and Mary; Inst. de Microelectronica de Madrid

2009-03-01

89

A novel image analysis methodology for online monitoring of nucleation and crystal growth during solid state phase transformations.  

PubMed

This study focuses on the development of an automated image analysis method to extract information on nucleation and crystal growth from polarized light micrographs. Using the developed image analysis method, four parameters related to nucleation and crystal growth could be extracted from the images. These parameters were crystalline count (applied as a measure of nucleation), percentage area coverage, average equivalent diameter and average crystalline area (three last parameters applied as a measure for crystal growth). The developed image analysis method was used to investigate two pharmaceutically relevant case studies: first, nitrendipine antisolvent crystallization, and second, recrystallization of amorphous piroxicam solid dispersion in an aqueous environment. In both case studies, an amorphous-to-crystalline phase transformation were observed, which were successfully monitored using real-time Raman spectroscopy. For the both case studies, the parameters related to crystallization kinetics estimated by image analysis were in close agreement with the parameters estimated by Raman spectroscopy. The developed image analysis method proved to be a valuable tool for quantitative monitoring of nucleation and crystal growth with an obvious potential for high throughput screening. PMID:22564780

Wu, Jian X; Xia, Dengning; van den Berg, Frans; Amigo, José Manuel; Rades, Thomas; Yang, Mingshi; Rantanen, Jukka

2012-05-04

90

Effects of the cosmological expansion on the bubble nucleation rate for relativistic first-order phase transitions  

NASA Astrophysics Data System (ADS)

I calculate the first corrections to the dynamical preexponential factor of the bubble nucleation rate for a relativistic first-order phase transition in an expanding cosmological background by estimating the effects of the Hubble expansion rate on the critical bubbles of Langer’s statistical theory of metastability. I also comment on possible applications and problems that arise when one considers the field theoretical extensions of these results (the Coleman De Luccia and Hawking-Moss instantons and decay rates).

Metaxas, Dimitrios

2008-09-01

91

Effects of the cosmological expansion on the bubble nucleation rate for relativistic first-order phase transitions  

SciTech Connect

I calculate the first corrections to the dynamical preexponential factor of the bubble nucleation rate for a relativistic first-order phase transition in an expanding cosmological background by estimating the effects of the Hubble expansion rate on the critical bubbles of Langer's statistical theory of metastability. I also comment on possible applications and problems that arise when one considers the field theoretical extensions of these results (the Coleman-De Luccia and Hawking-Moss instantons and decay rates)

Metaxas, Dimitrios [Department of Physics, National Technical University of Athens, Zografou Campus, 15780 Athens (Greece)

2008-09-15

92

Inclusion phases and the nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels  

Microsoft Academic Search

Series of submerged arc welds of HSLA steel made with three different fluxes and metallic additions of Ti, Mo, and Cr have\\u000a been examined to study the inclusions and their role in the nucleation of acicular ferrite. Inclusion phases and compositions\\u000a have been analyzed by electron diffraction and X-ray microanalysis. These analyses have shown that the inclusions contained\\u000a many different

J. M. Dowling; J. M. Corbett; H. W. Kerr

1986-01-01

93

Demagnetization via Nucleation of the Nonequilibrium Metastable Phase in a Model of Disorder  

NASA Astrophysics Data System (ADS)

We study both analytically and numerically demagnetization via nucleation of the metastable phase in a two-dimensional nonequilibrium Ising ferromagnet at temperature T. Canonical equilibrium is dynamically impeded by a weak random perturbation which models homogeneous disorder of undetermined source. We present a simple theoretical description, in good agreement with Monte Carlo simulations, assuming that the decay of the nonequilibrium metastable state is due, as in equilibrium, to the competition between the surface and the bulk. This suggests one to accept a nonequilibrium free-energy at a mesoscopic/cluster level, and it ensues a nonequilibrium surface tension with some peculiar low- T behavior. We illustrate the occurrence of intriguing nonequilibrium phenomena, including: (i) cooperative phenomena at low T which stabilize the metastable state as temperature increases; (ii) reentrance of the limit of metastability under strong nonequilibrium conditions; and (iii) noise-enhanced propagation of domain walls. We also studied metastability in the case of open boundaries as it may correspond to a magnetic nanoparticle. We then observe the most irregular relaxation triggered by the additional surface randomness. In particular, at low T, the relaxation becomes discontinuous as occurring by way of scale-free avalanches, so that it resembles the type of relaxation reported for many complex systems. We show that this results from the superposition of many demagnetization events, each with a well-defined scale which is determined by the curvature of the domain wall at which it originates. This is an example of (apparent) scale invariance in a nonequilibrium setting which is not to be associated with any familiar kind of criticality.

Hurtado, Pablo I.; Marro, J.; Garrido, P. L.

2008-10-01

94

Growth of gas phase nanoparticles with an accretion mechanism  

NASA Astrophysics Data System (ADS)

Nano-droplet growth in a supersaturated vapor has been investigated in a gas aggregation source using laser-ionization time-of-flight mass spectrometry. During its propagation into an atomic vapor, a small particle grows by sticking atoms on its surface. This accretion process has been highlighted through the clustering of homogeneous particles Mn and heterogeneous Mn(M2O) and Mn(MOH)2 particles in a metallic vapor and a helium buffer gas (M = Na or K). A modelization is introduced so as to connect the measured cluster mass distributions to the pertinent physical parameters. The mass distribution width is particularly sensitive to the efficiency of the first steps in the growth sequence. We used this property to compare the ability of this vapor-condensed matter phase transition to occur around various homogeneous and heterogeneous nucleation seeds.

Feiden, P.; Stehlé, J.; Leygnier, J.

2008-11-01

95

Comment on"Elucidating the Mechanism of Nucleation near the Gas-Liquid Spinodal"  

SciTech Connect

In a recent Letter [1], Bhimalapuram, Chakrabarty and Bagchi (BCB) study the phase transformation mechanism of the Lennard-Jones fluid and the non-conserved Ising model. They compute the free energy as a function of the size of the largest droplet of the stable phase. In apparent contradiction to classical nucleation theory (CNT), they find that in both systems the free energy develops a minimum at subcritical cluster sizes. In this Comment we argue that this minimum is specific to the chosen order parameter, and that the observed behavior is in fact consistent with CNT. CNT states that the free energy F(N) of a single cluster of size N is a concave function with a maximum at the critical nucleus size N{sub c}. BCB, on the other hand, calculate the probability distribution of N*, the size of the largest cluster in the system, and compute the free energy {beta}F*(N*) = -ln P(N*), where {beta} = 1/k{sub B}T. This order parameter does not measure the size of a single cluster. Instead, when sampling small values of N*, one measures the statistical weight of configurations in which all clusters are at most N* in size. Hence a free energy penalty is incurred when one constrains N* to values smaller than the largest average cluster in the simulation volume V. It is this penalty that causes the sudden increase of F* as N* {yields} 0 and the minimum at intermediate values of N*. We now illustrate how F(N) can be calculated from simulations. Our argument is intuitive but not exact, a formal derivation that yields an equivalent result can be found in Ref. 2. We choose the Ising model for concreteness. We aim to compute the probability that a given cluster has size N, where we imagine the center of the cluster to be fixed at site i. To simplify the calculation we consider clusters that overlap with site i, and correct for the N-fold translational degeneracy in a second step.

Maibaum, Lutz; Maibaum, Lutz

2008-06-18

96

Laser nucleation and solid phase crystallization of a-Si:H  

NASA Astrophysics Data System (ADS)

The ability to grow large-area, large-grained polycrystalline silicon on inexpensive substrates is becoming increasingly important for photovoltaic (PV) devices. With large-grained (grain size <10 ?m) 10 ?m thick films it is possible with light trapping to achieve PV efficiencies exceeding 15%. If crystallites could be nucleated and grown for longer times before native nucleation occurs, then potentially these much larger grain, thin film silicon material could be produced.The interaction of sub-crystallization threshold laser fluence with hydrogenated amorphous silicon (a-Si:H) has been demonstrated on a macroscopic scale to shorten the incubation time in subsequently thermally annealed films. Further examination of crystallite laser nucleation, found that nucleation was suppressed around PECVD a-Si:H thin film(50-100nm) sample edges, and scratches, in addition to laser-ablated areas, extending as much as 100-200 ?m laterally from these features. Optical microscopy and stepwise high temperature thermal annealing were used to investigate this behavior for the a-Si:H films deposited on glass substrates. The nucleation rates were measured in the treated and untreated regions. The data suggests that these features (edges, scratches, and laser ablated areas) provide stress relief by interrupting the surface connectivity. We confirm the existence of stress and stress relief by ?-Raman measurements of the crystallite transverse optical peak position relative to that of c-Si. PECVD films were annealed at temperatures between 540-600C, to enable a determination of rn at each anneal temperature. The temperature dependent measurements enabled the determination of the nucleation rate activation energies (EA), and how they are affected by film stress.

Dabney, M. S.; Mahan, A. H.; van Hest, M. F.; Ginley, D. S.

2012-10-01

97

Nucleation in ZBLAN glasses  

Microsoft Academic Search

Nucleation rates were measured in a ZrFâ-BaFâ-NaF-LaFâ-AlFâ glass (ZBLAN) using an optical technique. The results were compared with a similar glass having a slightly different composition. The difference in the nucleation rate is explained by classical nucleation theory using calculated free-energy differences between the ZBLAN liquid and the BaZrFâ crystal, which is found as the precipitating phase.

Gerard de Leede; H. deWaal; Frank Hakkens

1989-01-01

98

Nucleation of product phase in reactive diffusion of Al\\/Co  

Microsoft Academic Search

The reactive interdiffusion of Co\\/Al bilayers is studied by atom probe tomography. For that, metallic thin films were deposited on tungsten substrates preshaped by field evaporation. Owing to the outstanding resolution of the method and its real three-dimensional analysis, the nucleation of the first product could be characterized in detail. It is clearly seen that interdiffusion of the initial Al

V. Vovk; G. Schmitz; R. Kirchheim

2004-01-01

99

Nucleation of lamellar domains from a sponge phase under shear flow: Shape selection of nuclei in a nonequilibrium steady state  

NASA Astrophysics Data System (ADS)

It is a fundamental physical problem how a state is selected in a nonequilibrium steady state where the energy is continuously dissipated. This problem is common to phase transitions in liquids under shear flow and those in solids under deformation or electric current. In particular, soft matter often exhibits a strong nonlinear response to an external field, since its structural susceptibility to the external field is extremely large due to its softness and flexibility. Here we study the nucleation and growth process of the lamellar phase from the sponge phase under shear flow in a bilayer-forming surfactant system. We found an interesting shape selection of lamellar nuclei under shear flow between multilamellar vesicles (onions) and cylinders (leeks). These two types of behavior are separated sharply at a critical shear rate: a slight change of the shear rate is enough to switch one behavior to the other. We also found that, under a sufficiently strong shear flow, nucleated onions decrease their size with time, and eventually transform into leeks. This suggests that leeks may be the stable morphology under steady shear flow. However, the stability is limited only to the lamellar-sponge coexistence region. When a system enters into the lamellar phase region by further cooling, leeks lose their stability and break up into rather monodisperse onions, presumably via Rayleigh-like instability of a fluid tube. On the basis of these results, we draw a dynamic state diagram of smectic membrane organization under shear flow.

Miyazawa, Hideyuki; Tanaka, Hajime

2007-07-01

100

Ion irradiation-induced phase transformations in bixbyite-fluorite related oxides: The role of dislocation loop nucleation  

NASA Astrophysics Data System (ADS)

The ion irradiation induced phase transformations of bixbyite-fluorite related oxides have been studied with thin films of Y2O3 grown on Si and SrTiO3 by an ion beam sputtering technique which allows to control the microstructure and stresses within the film. Y2O3 thin films are ion-irradiated with an xenon beam in an energy range 60-380 keV at a fluence of 4.4 × 1015 Xe/cm2 at 80 K. Depending on the energy of the xenon beam two different structural phase transformations are observed: cubic to nanocrystalline/amorphous and cubic to monoclinic phase transformations. The phase transformation are analysed in terms of structural extended defect nucleation like prismatic dislocation loops due to the oxygen network behaviour under ion irradiation.

Gaboriaud, Rolly J.; Lacroix, Bertrand; Paumier, Fabien

2012-04-01

101

Effect of HCl addition on gas-phase and surface reactions during homoepitaxial growth of SiC at low temperatures  

Microsoft Academic Search

A complex influence of HCl addition on gas-phase and surface reactions during the low-temperature halo-carbon homoepitaxial growth of 4H-SiC was investigated. The addition of HCl was employed to reduce the undesirable effects of homogeneous gas-phase nucleation leading to formation of silicon clusters in the gas phase. It was established that dissociation of silicon clusters by HCl is efficient even at

G. Melnychuk; H. D. Lin; S. P. Kotamraju; Y. Koshka

2008-01-01

102

Phase transition in ?-nucleated isotactic polypropylene induced by combination of annealing and high pressure  

Microsoft Academic Search

The effects of annealing and high pressure on the microstructure of ?-nucleated isotactic polypropylene (iPP) were investigated. Annealing treatment was carried out at different temperatures from 90–170 °C under different pressure conditions, i.e. atmospheric pressure and high pressures of 150, 250, and 350 MPa, respectively. The microstructure of the specimens was comparatively investigated through using wide angle X-ray diffraction (WAXD),

Xiaoxi Li; Haiyan Wu; Gang Yang; Xiuru Liu; Shiming Hong; Yong Wang

2012-01-01

103

Quantification of gypsum crystal nucleation, growth, and breakage rates in a wet flue gas desulfurization pilot plant  

SciTech Connect

The aim of this work is to study the influence of nucleation, growth and breakage on the particle size distribution (PSD) of gypsum crystals produced by the wet flue gas desulfurization (FGD) process. The steady state PSD, obtained in a falling film wet FGD pilot plant during desulfurization of a 1000 ppm(V) SO{sub 2} gas stream, displayed a strong nonlinear behaviour (in a ln(n(l)) vs. I plot) at the lower end of the particle size range, compared to the well-known linear mixed suspension mixed product removal model. A transient population balance breakage model, fitted to experimental data, was able to model an increase in the fraction of small particles, but not to the extent observed experimentally. A three-parameter, size-dependent growth model, previously used for sodium sulphate decahydrate and potash alum, was able to describe the experimental data, indicating either size-dependent integration kinetics or growth rate dispersion.

Hansen, B.B.; Kiil, S.; Johnsson, J.E. [Technical University of Denmark, Lyngby (Denmark). Dept. of Chemical & Biochemical Engineering

2009-10-15

104

Activated carbon for gas phase arsenic capture  

Microsoft Academic Search

Investigation of activated carbon as a multifunctional sorbent for trace metal capture is the focus of this study. In addition to mercury and halides, selenium and arsenic represent two of the most volatile trace species that remain in gas phase in substantial amounts. In this work, fundamental sorption characteristics of the activated carbon for arsenic removal from the gas phase

R. Jadhav; H. Gupta; S. Misro; R. Agnihotri; L. S. Fan

1999-01-01

105

Gas evolution during water electrolysis  

Microsoft Academic Search

A survey is given of two-phase flow phenomena occurring during boiling and during electrolysis with gas evolution. The analogies and differences between bubble behavior in nucleate boiling and in nucleate electrolysis are discussed. Vapour bubble behavior during nucleate boiling of binary systems is governed by combined heat and mass diffusion. 50 refs.

S. J. D. Van Stralen; C. W. M. P. Sillen

1981-01-01

106

Gas-phase NO+ affinities  

PubMed Central

A scale of relative gas-phase NO+ binding energies (BEs) has been constructed by evaluation of NO+-transfer equilibria L1NO+ + L2 ? L2NO+ + L1 by Fourier-transform ion cyclotron resonance mass spectrometry and by application of the kinetic method, based on the metastable fragmentation of L1(NO+)L2 nitryl-ion bound dimers. The relative scale, anchored to the NO+ affinity of water, for 52 ligands, including alkyl halides, alkyl nitrates, alcohols, nitroalkanes, nitriles, aldehydes, ketones, and aromatic and heterocyclic compounds, led to an absolute NO+ affinity scale. The results are compared with those of an earlier study, and the apparent discrepancies are traced to a different choice of the absolute BE value used as the reference standard. The NO+ BEs fit a satisfactorily linear correlation when plotted versus the corresponding proton affinities (PAs). The NO+ BEs, while much lower than the PAs, are nevertheless higher than the corresponding BEs of the strictly related NO2+ cation, a result consistent with the experimental and theoretical results currently available on the structure and the stability of NO+ and NO2+ complexes. The NO+ BE vs. PA correlation allows one to estimate within 1–2 kcal·mol?1 the NO+ BE of the molecules included in the comprehensive PA compilations currently available. For example, the correlation gives the following NO+ affinities of the DNA bases, in kcal·mol?1 (1 kcal = 4.18 kJ): adenine, 40.3; cytosine, 40.4; guanine, 40.1; and thymine, 34.9. The experimental NO+ BE of thymine, the only one accessible to direct measurement, amounts to 35.6 ± 2 kcal·mol?1, which underlines the predictive value of the correlation. This study reports the second successful extension of the kinetic method to the evaluation of the absolute BEs of polyatomic cations, following our recent application to the strictly related NO2+ ion.

Cacace, F.; de Petris, G.; Pepi, F.

1997-01-01

107

Atmospheric nucleation: Measurements, mechanisms, and dynamics  

NASA Astrophysics Data System (ADS)

New particle formation by nucleation of gas-phase species significantly influences the size distributions and number concentrations of atmospheric aerosols. These nucleated particles are formed at rates that are orders of magnitude higher than were predicted by early models and grow at rates that are typically ten times faster than can be explained by the condensation of sulfuric acid alone. The resultant aerosols exert a significant impact on global climate by affecting the earth's radiation balance directly through the scattering and absorption of incident solar radiation, and indirectly through their role as cloud condensation nuclei (CCN). High formation rates and fast growth to CCN sizes ensure that NPF contributes significantly to the global CCN population. It is the primary goal of the research described in this thesis to develop robust models, constrained by measurement, for the sequential formation of CCN from the nucleation of gas-phase precursors. To this end, my thesis focuses on four topics: the development of nucleation rate parameterizations from correlations between formation rates of 1 nm particles and gas-phase sulfuric acid concentrations in diverse environments; the development of a cluster formation mechanism incorporating energetic barriers at the smallest clusters; the derivation of a simple, dimensionless criterion determining whether or not NPF would occur on a particular day; and the determination of the survival probability of newly formed particles (3 nm) as they grow to a CCN-active size (100 nm).

Kuang, Chongai

108

Solubility of aqueous methane under metastable conditions: implications for gas hydrate nucleation.  

PubMed

To understand the prenucleation stage of methane hydrate formation, we measured methane solubility under metastable conditions using molecular dynamics simulations. Three factors that influence solubility are considered: temperature, pressure, and the strength of the modeled van der Waals attraction between methane and water. Moreover, the naturally formed water cages and methane clusters in the methane solutions are analyzed. We find that both lowering the temperature and increasing the pressure increase methane solubility, but lowering the temperature is more effective than increasing the pressure in promoting hydrate nucleation because the former induces more water cages to form while the latter makes them less prevalent. With an increase in methane solubility, the chance of forming large methane clusters increases, with the distribution of cluster sizes being exponential. The critical solubility, beyond which the metastable solutions spontaneously form hydrate, is estimated to be ~0.05 mole fraction in this work, corresponding to the concentration of 1.7 methane molecules/nm(3). This value agrees well with the cage adsorption hypothesis of hydrate nucleation. PMID:23639139

Guo, Guang-Jun; Rodger, P Mark

2013-05-14

109

Nucleation and droplet growth at high pressure  

Microsoft Academic Search

Homogeneous nucleation, the first stage of droplet formation in the absence of foreign particles, usually takes place in the presence of one or more supercritical carrier gases. The present work aimed at a systematic investigation of the effects of carrier gas pressure on nucleation and droplet growth. Using the so-called nucleation pulse method, nucleation and droplet growth can be separated

Carlo Cornelis Maria Luijten

1998-01-01

110

Numerical study of the effect of gas temperature on the time for onset of particle nucleation in argon-silane low-pressure plasmas  

NASA Astrophysics Data System (ADS)

Particle nucleation in silane plasmas has attracted interest for the past decade, both due to the basic problems of plasma chemistry involved and the importance of silane plasmas for many applications. A better understanding of particle nucleation may facilitate the avoidance of undesirable particle contamination as well as enable the controlled production of nanoparticles for novel applications. While understanding of particle nucleation has significantly advanced over the past years, a number of questions have not been resolved. Among these is the delay of particle nucleation with an increasing gas temperature, which has been observed in experiments in argon-silane plasmas. We have developed a quasi-one-dimensional model to simulate particle nucleation and growth in silane containing plasmas. In this paper we present a comparative study of the various effects that have been proposed as explanations for the nucleation delay. Our results suggest that the temperature dependence of the Brownian diffusion coefficient is the most important effect, as diffusion affects both the loss rate and growth rate of particles.

Bhandarkar, Upendra; Kortshagen, Uwe; Girshick, Steven L.

2003-06-01

111

Atomic and Molecular Gas Phase Spectrometry.  

National Technical Information Service (NTIS)

The research involves an extensive investigation of analytical and physical aspects of gas phase spectroscopy. The goal of these studies is the development of high sensitivity, high selectivity, precise, accurate methods of analysis of elements in real sa...

J. D. Winefordner

1987-01-01

112

Sizes of nanobubbles from nucleation rate measurements  

NASA Astrophysics Data System (ADS)

In homogeneous bubble nucleation, the critical nucleus typically has nanometer dimensions. The volume V of a critical bubble can be determined from the simple equation (partial W/partial p)_T=V, where W is the reversible work of nucleus formation and p is the ambient pressure of the liquid phase in which bubble formation is occurring. The relation, W/kT=-ln J+ln A, where J is the steady state nucleation rate and A is the weakly pressure-dependent kinetic prefactor, allows V to be determined from rate measurements. The original derivation of this equation for V from the nucleation theorem was limited to one-component, ideal gas bubbles with a gas density much smaller than that of the ambient liquid. [D. Kashchiev, Nucleation: basic theory with applications (Butterworth-Heinemann, Oxford, 2000) p. 226.] The result is actually much more general, and it will be shown that it applies to multi-component, nonideal gas bubbles, provided the same density inequality holds. When the bubble phase and liquid densities are comparable, a more complicated, but also general and rigorous result is found.

Wilemski, G.

2003-03-01

113

Condensed phase decomposition and gas phase combustion of hydrazinium nitroformate  

Microsoft Academic Search

This paper presents the results of a series of experiments on the condensed phase decomposition and the gas phase combustion of hydrazinium nitroformate (HNF). The experiments include SEM analysis of quenched samples that showed evidence of the formation of a foam layer. FTIR spectrometry and mass spectrometry provide details on species formation during decomposition and combustion. The analysis of the

O. E. Dragomir; M. J. Tummers; E. H. van Veen; A. E. D. M. van der Heijden; D. J. E. M. Roekaerts

2009-01-01

114

Nucleation in ZBLAN glasses  

SciTech Connect

Nucleation rates were measured in a ZrF{sub 4}-BaF{sub 2}-NaF-LaF{sub 3}-AlF{sub 3} glass (ZBLAN) using an optical technique. The results were compared with a similar glass having a slightly different composition. The difference in the nucleation rate is explained by classical nucleation theory using calculated free-energy differences between the ZBLAN liquid and the BaZrF{sub 6} crystal, which is found as the precipitating phase.

Leede, G.d.; deWaal, H.; Hakkens, F. (Eindhoven Univ. of Technology, Eindhoven (NL))

1989-10-01

115

Bin-Resolved Microphysical Modeling of Water Vapor Isotopic Composition During Ice Nucleation and Growth in Mixed-Phase Clouds  

NASA Astrophysics Data System (ADS)

Because any phase change in water is accompanied by isotopic fractionation, ice nucleation and growth in mixed-phase and cirrus clouds should be accompanied by large isotopic changes, and isotopic composition can be a useful tracer for understanding microphysics in these contexts. However, detailed microphysical modeling that includes isotopologues of water has not previously been done. We present here initial studies with the BRIMM ("Bin-Resolved Isotopic Microphysical Model") model, developed at ETH Zurich, a bin-resolved model that incorporates isotopic accounting during evaporation and condensation. Explicit resolution of particle sizes is especially important for isotopic microphysics because particle sizes affect isotopic evolution: in particular, heating via the latent heat of condensation has strong effects on isotopic fractionation and can be misrepresented by a bulk microphysical model. We show here the range of isotopic evolution produced by changes in physical parameters and processes, focusing especially on latent heat effects in the Bergeron process, as water is transferred from liquid to solid particles through the vapor phase. These isotopic studies can form the foundations for interpretation of eventual in-situ measurements of isotopic compositions in cirrus and mixed-phase clouds.

Aho, S.; Weinhold, F.; Peter, T.; Moyer, E. J.

2011-12-01

116

Gas Phase Diagnostics in LAPPS  

NASA Astrophysics Data System (ADS)

NRL is developing a scalable Large Area Plasma Processing System for materials processing applications.^a Using a magnetically collimated sheet of high energy electrons (2-5kV, 10 mA/cm^2) to ionize a neutral background gas, a high density plasma of comparable size with cold electrons and ions is produced independent of chamber configuration. This beam ionization process is readily scalable (to meters^2) with the limiting factor being the range of the electron beam in the background gas ( ~ 10 eV/cm). Presently, operating pressures range from 10-500 mtorr with 0-300 Gauss magnetic fields, using a hollow cathode as an electron beam source. Temporally and spatially resolved data from Langmuir probes, optical emission, microwave absorption/transmission and electron beam analysis of the plasma sheet in recent pulsed operation studies (10-1000 ?s pulse length, < 10 kHz pulse repetition frequency) will be presented for various gas mixtures. Ion densities from 10^9 to 5x10^12 cm-3 are obtained in plasma volumes of 1x30x30cm^3 in mixtures of oxygen, nitrogen, argon and neon. Overviews and additional details of the LAPPS process will be presented by co-authors.footnote See presentations by R. A. Meger, R. F. Fernsler, S. G. Walton and D. P. Murphy at this conference.

Leonhardt, D.; Walton, S. G.; Murphy, D. P.; Amatucci, W. E.; Meger, R. A.; Fernsler, R. F.

1999-10-01

117

Rupture initiation of the large subduction earthquakes: are the durations and moments of nucleation phases correlated with the final seismic moments? (Invited)  

NASA Astrophysics Data System (ADS)

Previous studies of near-source observations showed that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event. The duration and the moment of the nucleation phase are correlated with the final scalar moment (e.g., Ellsworth and Beroza, 1995). Here we evaluate it with the teleseismic P wave data of 10 Mw>8 subduction earthquakes and 2 Mw>8 outer-rise normal events since 2000. If the nucleation phases of these events also release ~0.5% of final total seismic moments (Ellsworth and Beroza, 1995), their moment magnitudes should be 6 or larger, being detectable even at teleseismic distances. To take advantage of the dense global and regional seismic networks, after aligning the similar teleseismic P waveforms of each event by waveform cross-correlation technique, they are stacked together to increase signal-noise ratio. Our preliminary results show that the nucleation phase indeed exists in most of events. Their durations vary significantly from nearly zero to over 10 s, but uncorrelated with the corresponding final seismic moments.

Ji, C.; Li, X.; Shao, G.

2010-12-01

118

Effect of Pressure on Phase Selection during Nucleation in Undercooled Bismuth.  

National Technical Information Service (NTIS)

At a sufficiently large liquid undercooling, the solidification of fine Bi droplet samples at ambient pressure yields a metastable phase instead of the stable structure. The metastable Bi phase is observed to melt at 174 C at ambient pressure. Thermal ana...

W. Yoon J. S. Paik D. LaCourt J. H. Perepezko

1986-01-01

119

Nucleation at the phase transition near 40 deg. C in MnAs nanodisks  

SciTech Connect

The phase transition near 40 deg. C of both as-grown thin epitaxial MnAs films prepared by molecular beam epitaxy on GaAs(001) and nanometer-scale disks fabricated from the same films is studied. The disks are found to exhibit a pronounced hysteresis in the temperature curve of the phase composition. In contrast, supercooling and overheating take place far less in the samples of continuous layers. These phenomena are explained in terms of the necessary formation of nuclei of the other phase in each of the disks independent from each other. The influence of the elastic strains in the disks is reduced considerably.

Jenichen, B.; Takagaki, Y.; Ploog, K. H.; Darowski, N.; Feyerherm, R.; Zizak, I. [Paul-Drude-Institut fuer Festkoerperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin (Germany); Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D-14109 Berlin (Germany)

2006-07-31

120

Link between liquid structure and the nucleation barrier for icosahedral quasicrystal, polytetrahedral, and simple crystalline phases in Ti-Zr-Ni alloys: Verification of Frank's hypothesis  

NASA Astrophysics Data System (ADS)

Comprehensive undercooling experiments on a large number of simple crystalline, polytetrahedral, and icosahedral quasicrystalline phase forming compositions in Ti-Zr-Ni alloys have been carried out using electrostatic levitation (ESL) techniques for containerless processing. Consistent with Frank’s hypothesis, a direct correlation was found between the reduced undercooling [ ?Tr=(Tl-Tr)/Tl , where Tr and Tl are the nucleation and liquidus temperatures, respectively] and the icosahedral short-range order in the solid. The reduced undercooling is less for liquids that form the icosahedral quasicrystal ( i phase) than for those that form the hcp C14 Laves polytetrahedral phase. For many compositions near 21at.% Ni, the primary nucleation of a metastable i phase instead of a stable C14 Laves phase demonstrates that the interfacial free energy between the liquid and the i phase is smaller than between the liquid and the C14 Laves phase, indicating icosahedral local order in the undercooled liquid. This is in agreement with a classical-nucleation-theory-based estimate of the interfacial free energy and the work of formation of the critical cluster from the undercooling data. Taken together with high-energy x-ray diffraction studies of the undercooled liquid, these results demonstrate that the local structure of liquids in Ti-Zr-Ni alloys is icosahedral, as postulated by Frank over a half century ago.

Lee, G. W.; Gangopadhyay, A. K.; Croat, T. K.; Rathz, T. J.; Hyers, R. W.; Rogers, J. R.; Kelton, K. F.

2005-11-01

121

Nucleation mechanism of the sigma -to- alpha phase transition in Fe1-xCrx  

Microsoft Academic Search

This Rapid Communication reports on Mössbauer-spectroscopy measurements of the kinetics of the sigma -to- alpha phase transition in the system Fe-Cr. By isothermally annealing samples of the compositions Fe53.8Cr46.2 and Fe51Cr49 at temperatures between 820 and 855°C the authors gained information about the kinetics of the phase transition in terms of the Johnson-Mehl-Avrami-Kolmogorov equation. The obtained values for the Avrami

Alice Mikikits-Leitner; Bogdan Sepiol; Michael Leitner; Jakub Cieslak; Stanislaw M. Dubiel

2010-01-01

122

Sigma-phase equilibria and nucleation in Fe-Cr-Ni alloys at high temperature  

Microsoft Academic Search

The sigma-phase is of crucial importance because of the disadvantageous influence on mechanical properties of the commercial alloys, mainly lowering the impact toughness and reducing creep ductility. It is usually assumed that the sigma-phase forms in the iron based alloys which contain the high level Cr and the formation is not possible above 1,104 K and below about 713 K

J. Sopoušek; T. Krurnl

1996-01-01

123

In-plane bidimensional stripes in thin films of CCH-PCH mixtures: possible nucleation of a tilted smectic phase  

NASA Astrophysics Data System (ADS)

Spontaneous in-plane bidimensional stripes are predicted to appear in thin films of tilted smectic liquid crystals, due to flexopolarization, provided the flexoelectric coefficients are great enough, as in several smectic C's. The two periodicities arise in orthogonal directions. In the presence of free ions, the charge distributions becomes also modulated in-plane, partially masking the direct flexopolarization influence. Both effective periods are of the order of 10 micrometers . The optical observations were performed on mixtures of trans-4-(trans-4'-n- amylcyclohexyl) cyclohexanecarboxylic acid (CCH) and of 4-(trans-4'-n-amylcyclohexyl) benzoic acid (PCH). Both pure compounds exhibit only a nematic phase, the first between 224 degree(s)C and 243 degree(s)C, and the second one between 180 degree(s)C and 265 degree(s)C. The experimental data are consistent with the present model, suggesting that, for certain CCH/PCH ratios less than 30%, a tilted smectic phase is nucleated close to the melting point.

Pikin, Sergei A.; Torgova, Sonja I.; Strigazzi, Alfredo

1996-01-01

124

Isothermal nucleation and growth kinetics of Pd/Ag alloy phase via in-situ time-resolved high-temperature x-ray diffraction (HTXRD) analysis  

SciTech Connect

Among several different approaches to form Pd/Ag alloys for hydrogen separation applications, ex-situ studies carried by conventional X-ray point scanning detectors might fail to reveal the key aspects of the phase transformation between Pd and Ag metals. In this respect, in-situ time-resolved high temperature X-ray diffraction (HTXRD) was employed to study the Pd/Ag alloy phase nucleation and growth kinetics. By the use of linear position sensitive detectors, advanced optics and profile fitting with the use of JADE-6.5 software, isothermal phase evolution of the Pd/Ag alloy at 500 C, 550 C and 600 C under hydrogen atmosphere were quantified to elucidate the mechanistic details of the Pd/Ag alloy phase nucleation and growth pattern. Analysis of the HTXRD data by the Avrami model indicated that the nucleation of the Pd/Ag alloy phase was instantaneous where the growth mechanism was through diffusion-controlled one-dimensional thickening of the Pd/Ag alloy layer. The value of the Avrami exponent, n, was found to increase with temperature with the values of 0.34, 0.39 and 0.67 at 500oC, 550oC and 600oC, respectively. In addition, parabolic rate law analysis suggested that the nucleation of the Pd/Ag alloy phase was through a heterogeneous nucleation mode, in which the nucleation sites were defined as the non-equilibrium defects. The cross-sectional SEI micrographs indicated that the Pd/Ag alloy phase growth was strongly dependent upon the deposition morphology of the as-synthesized Pd and Ag layers formed by the electroless plating. Based on the Avrami model and the parabolic rate law, the estimated activation energies for the phase transformation were 236.5 and 185.6 kJ/mol and in excellent agreement with the literature values (183-239.5 kJ/mol).

Ayturk, Mahmut Engin [Worcester Polytechnic Institute; Payzant, E Andrew [ORNL; Speakman, Scott A [ORNL; Ma, Yi Hua [Worcester Polytechnic Institute

2008-01-01

125

Infrared spectroscopy of homogeneously nucleated hydrazine aerosols - Disordered and crystalline phases  

NASA Astrophysics Data System (ADS)

It is shown that aerosols generated at low temperatures and high condensation rate spontaneously form in a highly crystalline state. The resonant absorption bands in the IR spectra of these highly crystalline particles are much sharper than any reported previously in the bulk, and reveal details in the N-H vibrational bands that have not been previously observed. A disordered phase is also observed at somewhat higher temperatures. These results are consistent with this being a supercooled liquid. The fact that the spectra associated with these two aerosol phases are quite different is important to any future attempts at detecting hydrazine aerosols in planetary atmospheres by remote sensing techniques.

Dunder, T.; Clapp, M. L.; Miller, R. E.

1993-01-01

126

EPA GAS PHASE CHEMISTRY CHAMBER STUDIES  

EPA Science Inventory

Gas-phase smog chamber experiments are being performed at EPA in order to evaluate a number of current chemical mechanisms for inclusion in EPA regulatory and research models. The smog chambers are 9000 L in volume and constructed of 2-mil teflon film. One of the chambers is co...

127

Homogeneous nucleation of nitrogen  

NASA Astrophysics Data System (ADS)

We investigated the homogeneous nucleation of nitrogen in a cryogenic expansion chamber [A. Fladerer and R. Strey, J. Chem. Phys. 124, 164710 (2006)]. Gas mixtures of nitrogen and helium as carrier gas were adiabatically expanded and cooled down from an initial temperature of 83 K until nucleation occurred. This onset was detected by constant angle light scattering at nitrogen vapor pressures of 1.3-14.2 kPa and temperatures of 42-54 K. An analytical fit function well describes the experimental onset pressures with an error of +/-15%. We estimate the size of the critical nucleus with the Gibbs-Thomson equation yielding critical sizes of about 50 molecules at the lowest and 70 molecules at the highest temperature. In addition, we estimate the nucleation rate and compare it with nucleation theories. The predictions of classical nucleation theory (CNT) are 9 to 19 orders of magnitude below the experimental results and show a stronger temperature dependence. The Reguera-Reiss theory [Phys. Rev. Lett. 93, 165701 (2004)] predicts the correct temperature dependence at low temperatures and decreases the absolute deviation to 7-13 orders of magnitude. We present an empirical correction function to CNT describing our experimental results. These correction parameters are remarkably close to the ones of argon [Iland et al., J. Chem. Phys. 127, 154506 (2007)] and even those of water [J. Wölk and R. Strey, J. Phys. Chem. B 105, 11683 (2001)].

Iland, Kristina; Wedekind, Jan; Wölk, Judith; Strey, Reinhard

2009-03-01

128

Activated carbon for gas phase arsenic capture  

SciTech Connect

Investigation of activated carbon as a multifunctional sorbent for trace metal capture is the focus of this study. In addition to mercury and halides, selenium and arsenic represent two of the most volatile trace species that remain in gas phase in substantial amounts. In this work, fundamental sorption characteristics of the activated carbon for arsenic removal from the gas phase are investigated. Activated carbons with different structural properties are studied for their usefulness in removing arsenic species from flue gas. Arsenic oxide (As{sub 2}O{sub 3}) is used as the source of arsenic. Preliminary sorption studies indicate that arsenic removal occurs by physical adsorption, with increased capture by carbons with higher surface areas.

Jadhav, R.; Gupta, H.; Misro, S.; Agnihotri, R.; Fan, L.S.

1999-07-01

129

Nucleation and growth of Ag islands on the (.sqroot.3 × .sqroot.3)R30° phase of Ag on Si(111)  

SciTech Connect

We use scanning tunneling microscopy to measure densities and characteristics of Ag islands that form on the ({radical}3 x {radical}3)R30{sup o}-Ag phase on Si(111), as a function of deposition temperature. Nucleation theory predicts that the logarithm of island density varies linearly with inverse deposition temperature. The data show two linear regimes. At 50-125 K, islands are relatively small, and island density decreases only slightly with increasing temperature. At 180-250 K, islands are larger and polycrystalline, and island density decreases strongly with increasing temperature. At 300 K, Ag atoms can travel for distances of the order of 1 {micro}m. Assuming that Ag diffusion occurs via thermally activated motion of single atoms between adjacent sites, the data can be explained as follows. At 50-125 K, the island density does not follow conventional Arrhenius scaling due to limited mobility and a consequent breakdown of the steady-state condition for the adatom density. At {approx} 115-125 K, a transition to conventional Arrhenius scaling with critical nucleus size (i = 1) begins, and at 180-250 K, i > 1 prevails. The transition points indicate a diffusion barrier of 0.20-0.23 eV and a pairwise Ag-Ag bond strength of 0.14 eV. These energy values lead to an estimate of i {approx} 3-4 in the regime 180-250 K, where island density varies strongly with temperature.

Belianinov, A.; Unal, B.; Ho, K.-M.; Wang, C.-Z.; Evans, J. W.; Tringides, M. C.; Thiel, P. A.

2011-06-06

130

CARBON DIOXIDE SEPARATION BY PHASE ENHANCED GAS-LIQUID ABSORPTION  

SciTech Connect

A new process called phase enhanced gas-liquid absorption has been developed in its early stage. It was found that adding another phase into the absorption system of gas/aqueous phase could enhance the absorption rate. A system with three phases was studied. In the system, gas phase was carbon dioxide. Two liquid phases were used. One was organic phase. Another was aqueous phase. By addition of organic phase into the absorption system of CO{sub 2}-aqueous phase, the absorption rate of CO{sub 2} was increased significantly. CO{sub 2} finally accumulated into aqueous phase. The experimental results proved that (1) Absorption rate of carbon dioxide was enhanced by adding organic phase into gas aqueous phase system; (2) Organic phase played the role of transportation of gas solute (CO{sub 2}). Carbon dioxide finally accumulated into aqueous phase.

Liang Hu; Adeyinka A. Adeyiga

2004-05-01

131

CARBON DIOXIDE SEPARATION BY PHASE ENHANCED GAS-LIQUID ABSORPTION  

SciTech Connect

A new process called phase enhanced gas-liquid absorption has been developed in its early stage. It was found that adding another phase into the absorption system of gas/aqueous phase could enhance the absorption rate. A system with three phases was studied. In the system, gas phase was carbon dioxide. Two liquid phases were used. One was organic phase. Another was aqueous phase. By addition of organic phase into the absorption system of CO{sub 2}-aqueous phase, the absorption rate of CO{sub 2} was increased significantly. CO{sub 2} finally accumulated into aqueous phase. The experimental results proved that (1) Absorption rate of carbon dioxide was enhanced by adding organic phase into gas aqueous phase system; (2) Organic phase played the role of transportation of gas solute (CO{sub 2}). Carbon dioxide finally accumulated into aqueous phase.

Liang Hu

2004-09-30

132

From gas-phase oxidation of SO2 by SO4- to the formation of sulfuric acid  

NASA Astrophysics Data System (ADS)

One of the difficulties to predict atmospheric nucleation is related to inaccurate measure of the total sulfuric acid concentration. We present a density functional theory investigation of the SO2 gas phase oxidation by SO4-. In the immediate product, SO2.SO4- cluster, SO2 is subsequently oxidized and SO3SO3- is formed at 1.7 × 10-7 s-1 reaction rate. SO3SO3- interacts with O2 molecule to form SO3 and SO5-, which are important species in the gas phase chemistry of sulfur and in the formation mechanism of sulfuric acid.

Tsona, Narcisse; Bork, Nicolai; Vehkamäki, Hanna

2013-05-01

133

A time-resolved numerical study of the vapor-liquid-solid growth kinetics describing the initial nucleation phase as well as pulsed deposition processes.  

PubMed

Today, the vapor-liquid-solid (VLS) growth mechanism is a common process for the metal catalyzed bottom-up growth of semiconductor nanowires (NWs). Nevertheless, most of the literature only is concerned with the steady-state NW growth which applies when the amount of material supplied is equal to the amount consumed by the NW growth at the same time. While this description is suitable for chemical vapor deposition (CVD) or electron beam evaporation (EBE) processes after the initial nucleation time, problems arise when pulsed growth processes like pulsed laser deposition (PLD) are used since in this case the steady state growth condition cannot be applied. Moreover, the initial phase of NW growth cannot be described with steady state growth conditions, either. In this work, we present a modeling approach for VLS NW growth based on numerical simulations, which is capable of describing the nucleation phase of the VLS growth process as well as a pulsed deposition process. PMID:23394587

Eisenhawer, Björn; Sivakov, Vladimir; Christiansen, Silke; Falk, Fritz

2013-02-18

134

Excimer laser and rapid thermal annealing stimulation of solid-phase nucleation and crystallization in amorphous silicon films on glass substrates  

Microsoft Academic Search

The solid-phase crystallization process in thin amorphous silicon films on glass substrates was studied with application of excimer laser annealing (ELA) and rapid thermal annealing (RTA) for stimulation of nucleation. Use of ELA allowed us to create homogeneous polycrystalline silicon films on glass with grain sizes up to 0953-8984\\/8\\/3\\/007\\/img8 at temperatures below 0953-8984\\/8\\/3\\/007\\/img9C. Use of RTA reduced the incubation time

M. D. Efremov; V. V. Bolotov; V. A. Volodin; L. I. Fedina; E. A. Lipatnikov

1996-01-01

135

An in vitro Study of a Phase-Shift Nanoemulsion: A Potential Nucleation Agent for Bubble-Enhanced HIFU Tumor Ablation  

Microsoft Academic Search

Phase-shift nanoemulsions have the potential to nucleate bubbles and enhance high-intensity focused ultrasound (HIFU) cancer therapy. This emulsion consists of albumin-coated dodecafluoropentane (DDFP) droplets with a mean diameter of approximately 260 nm at 37°C. It is known that superheated perfluorocarbon droplets can be vaporized with microsecond long ultrasound pulses if the acoustic pressure exceeds a specific threshold. In addition, it

Peng Zhang; Tyrone Porter

2010-01-01

136

Nonextensive nuclear liquid-gas phase transition  

NASA Astrophysics Data System (ADS)

We study an effective relativistic mean-field model of nuclear matter with arbitrary proton fraction at finite temperature in the framework of nonextensive statistical mechanics, characterized by power-law quantum distributions. We investigate the presence of thermodynamic instability in a warm and asymmetric nuclear medium and study the consequent nuclear liquid-gas phase transition by requiring the Gibbs conditions on the global conservation of baryon number and electric charge fraction. We show that nonextensive statistical effects play a crucial role in the equation of state and in the formation of mixed phase also for small deviations from the standard Boltzmann-Gibbs statistics.

Lavagno, A.; Pigato, D.

2013-10-01

137

Initiation of the ice phase by marine biogenic surfaces in supersaturated gas and supercooled aqueous phases.  

PubMed

Biogenic particles have the potential to affect the formation of ice crystals in the atmosphere with subsequent consequences for the hydrological cycle and climate. We present laboratory observations of heterogeneous ice nucleation in immersion and deposition modes under atmospherically relevant conditions initiated by Nannochloris atomus and Emiliania huxleyi, marine phytoplankton with structurally and chemically distinct cell walls. Temperatures at which freezing, melting, and water uptake occur are observed using optical microscopy. The intact and fragmented unarmoured cells of N. atomus in aqueous NaCl droplets enhance ice nucleation by 10-20 K over the homogeneous freezing limit and can be described by a modified water activity based ice nucleation approach. E. huxleyi cells covered by calcite plates do not enhance droplet freezing temperatures. Both species nucleate ice in the deposition mode at an ice saturation ratio, S(ice), as low as ~1.2 and below 240 K, however, for each, different nucleation modes occur at warmer temperatures. These observations show that markedly different biogenic surfaces have both comparable and contrasting effects on ice nucleation behaviour depending on the presence of the aqueous phase and the extent of supercooling and water vapour supersaturation. We derive heterogeneous ice nucleation rate coefficients, J(het), and cumulative ice nuclei spectra, K, for quantification and analysis using time-dependent and time-independent approaches, respectively. Contact angles, ?, derived from J(het)via immersion freezing depend on T, a(w), and S(ice). For deposition freezing, ? can be described as a function of S(ice) only. The different approaches yield different predictions of atmospheric ice crystal numbers primarily due to the time evolution allowed for the time-dependent approach with implications for the evolution of mixed-phase and ice clouds. PMID:21912788

Alpert, Peter A; Aller, Josephine Y; Knopf, Daniel A

2011-09-12

138

Experimental Thermochemistry of Gas Phase Cytosine Tautomers  

NASA Astrophysics Data System (ADS)

Enthalpies of interconversion are measured for the three lowest energy tautomers of isolated cytosine. The equilibrium distribution of tautomers near 600 K is frozen upon the capture of the gas phase species by low temperature helium nanodroplets. The temperature dependence of the gas phase cytosine tautomer populations is determined with infrared laser spectroscopy of the helium solvated species. The interconverison enthalpies obtained from the van't Hoff relation are 1.14 ± 0.21 and 1.63 ± 0.12 for the C31 rightleftharpoons C32 and C31 rightleftharpoons C1 equilibria, respectively. C31 and C32 are rotamers of an enol tautomer, and C1 is a keto tautomer. The interconversion enthalpies are compared to recent CCSD(T) thermochemistry calculations of cytosine tautomers.

Morrison, A. M.; Douberly, G. E.

2011-06-01

139

The gas-phase acidity of nitrocyclopropane  

NASA Astrophysics Data System (ADS)

Nitrocyclopropane is 10.5 kcal mol-1 weaker as an acid in the gas phase than its open-chain analog, 2-nitropropane. This is attributed to the conflicting hybridization requirements for carbanion stabilization by the cyclopropyl ring and by the nitro group. Based on reactivities, the deprotonated form does not ring-open to either the 2-nitroallyl anion or the 1-nitroallyl anion.

Bartmess, John E.; Wilson, Burton; Sorensen, Daniel N.; Bloor, John E.

1992-09-01

140

Gas-phase chemical sensing using electrochemiluminescence  

Microsoft Academic Search

Tris (2,2?-bipyridine) ruthenium (III) (Ru(bpy)33+), an electrogenerable, chemiluminescent reagent which is electrochemically reversible, has been demonstrated to be a suitable reagent for the sensitive detection of gas-phase hydrazine, and its derivatives dimethyl hydrazine and monomethyl hydrazine. The electrochemical oxidation of Ru(bpy)32+ to Ru(bpy)33+ creates a strong oxidizer, whose reduction back down to Ru(bpy)32+ by reaction with areducing analyte is typified

Greg E Collins

1996-01-01

141

Gas-Phase Photoionization Of A Protein  

NASA Astrophysics Data System (ADS)

We present preliminary results on gas phase photoionization of electrosprayproduced multiply protonated cytochrome c protein (104 amino acids; Ë?12.4 kDa), which has been achieved with a newly developed experimental system for spectroscopy of electrosprayed ions in a linear quadrupole ion trap using a monochromatized vacuum ultraviolet (VUV) synchrotron radiation and tandem mass spectrometry method. The investigation of proteins in the gas phase, where they are free of the influence of counterions and solvent molecules, offer a possibility to understand their intrinsic molecular properties. However, due to limited both ion densities and available number of photons, the use of synchrotron radiation for the trapped ions spectroscopy is a rather challenging task. The feasibility of coupling a Fourier transform ion cyclotron resonance ion trap with soft x-ray synchrotron beamline and the first successful use of synchrotron radiation for spectroscopy of electrosprayed negative ions stored in a three-dimensional quadrupole ion trap have been demonstrated only recently (R. Thissen et al., 2008, Phys. Rev. Lett., 100, 223001; A. Giulliani et al., Proc. 57th ASMS Conf., Philadelphia, 2009). The present results are the first reported on photoionization of kDa species in the gas phase and are valuable regarding both a fundamental interest of accessing physical properties of large biological ions isolated in vacuo and potential development of a new technique for proteomics.

Milosavljevic, A. R.; Giuliani, A.; Nicolas, C.; Gil, J.-F.; Lemaire, J.; Refregiers, M.; Nahon, L.

2010-07-01

142

Dependence of nucleation rates on sulfuric acid vapor concentration in PoValley, Italy  

NASA Astrophysics Data System (ADS)

A field campaign was conducted at the polluted rural site, San Pietro Capofiume (SPC) in PoValley, Italy, from June 26th to July 12th 2009 in the framework of the EUCAARI (European integrated project on aerosol, cloud, climate, and air interactions) project. The gas-phase sulfuric acid concentrations were measured for the first time at SPC station during this campaign. Here we examine the dependence of nucleation rate on sulfuric acid vapor concentrations in SPC. The apparent nucleation rate - i.e. formation rate of 3 nm particles - was directly determined from differential mobility particle sizer data. The nucleation rate at 1 nm (J1) was then calculated by accounting for the coagulation of the sub-3 nm clusters with large particles during their growth from 1 to 3 nm. The dependence of J1 on [H2SO4] was studied for each nucleation day individually and for all nucleation days as a whole. It is usually assumed that the dependence of nucleation rate on sulfuric acid concentration follows a simple power law model J1 = P [H2SO4]^n; where P is the prefactor containing chemical and physical information of the nucleation process, and n is the nucleation exponent. With the so called activation and kinetic nucleation mechanisms, n takes the values 1 and 2, respectively, and most field studies show n to fall between these two values. For the SPC data, the nucleation exponent was higher than 2 for both individual days and for all nucleation days as a whole. In the light of the obtained results we will discuss the nucleation mechanism, composition of the nucleation mode particles, and the role of other gases (such as ammonia and water vapor) in the formation of new particles in SPC in more detail.

Hamed, Amar; Plaß-Dülmer, Christian; Elste, Thomas; Stange, Georg; Decesari, Stefano; Carbone, Claudio; Facchini, Maria Cristina; Joutsensaari, Jorma; Laaksonen, Ari

2010-05-01

143

Characteristics of Nucleation Using the Bias-Enhanced Nucleation Method at Low Pressure  

NASA Astrophysics Data System (ADS)

The bias-enhanced nucleation (BEN) method is well known as a pretreatment method for nucleation, and the nucleus is generated in the plasma sheath region during the BEN process. To utilize high-density nucleation at low pressure and to expand the nucleation region in hot-filament chemical vapor deposition (HFCVD), the BEN method was applied under a low-pressure condition in which the plasma and plasma sheath expanded. Some substrates were treated in hydrogen plasma or activated hydrogen before nucleation. The purpose of this study is to confirm the characteristics of nucleation at low pressure and to determine the condition for realizing uniform, high-density and large-area nucleation. By using emitted electrons, bias voltage and current were controlled to be steady at the low pressure of 0.1 Torr. The nucleation density increased and the width of the nucleation area expanded when the substrate surface was treated in hydrogen plasma or activated hydrogen before nucleation. It was proposed that the nucleation mechanism is as follows. (1) A substrate surface was treated by H ions and H radicals, (2) a nucleation site was formed on the treated position and (3) nucleation occurred on the nucleation site. To achieve high-density and large-area nucleation, it is necessary to obtain the treated substrate surface and to generate carbon-containing cations, H ions and H radicals with appropriate ratios and density distributions in the nucleation phase.

Kudo, Kouichi; Aoki, Kousuke; Toyofuku, Masaharu; Ogi, Sukeomi

2001-04-01

144

Substrate-free gas-phase synthesis of graphene  

NASA Astrophysics Data System (ADS)

Graphene is a single atomic layer of sp2-bonded carbon atoms tightly packed in a two-dimensional honeycomb lattice. The material possesses remarkable properties and has been envisioned for use in numerous applications. Contemporary graphene production techniques require substrates or graphite crystals to create graphene. Furthermore, these approaches involve multiple steps, and sometimes non-ambient conditions, to produce atomically-thin sheets. This dissertation presents the first substrate-free gas-phase graphene synthesis method. The technique can synthesize graphene in a single step at atmospheric pressure, without the use of graphite or substrates. The novel synthesis method was discovered through experiments that tested the hypothesis that graphene could be synthesized through the delivery of alcohols into argon plasmas. The experiments presented in this dissertation were conducted in an atmospheric-pressure microwave plasma reactor. Solid carbon materials were produced by delivering liquid ethanol droplets directly into argon plasmas. Numerous characterization techniques were used to unambiguously prove that the synthesized materials were clean and highly ordered graphene sheets. Additional studies investigated the effects of variable experimental parameters on the graphene synthesis process. The applied microwave power did not significantly affect the types of structures produced in the reactor. Lowering the volumetric flow rate of the plasma gas resulted in the synthesis of graphitic particles. The composition of the precursors delivered into the reactor also affected graphene synthesis. Graphene was not produced through the delivery of methanol or isopropyl alcohol droplets. However, graphene was obtained through dimethyl ether, which is an organic compound with the same atomic composition as ethanol. Thus, the flow rate and precursor composition significantly affected the nucleation, growth, and residence time of the materials created during experiments. A practical application for the synthesized graphene is also presented in this dissertation. The sheets were found to be an ideal support structure for the transmission electron microscopy characterization of nanoparticles coated with molecular layers. The substrate-free gas-phase method is capable of rapid and continuous graphene synthesis at ambient pressure. The simplicity of the approach makes it scalable for industrial graphene production. The novel technique presented in this dissertation could substantially enable graphene research and applications.

Dato, Albert Manglallan

145

Aerosol Nucleation Rates of Sulfuric Acid and Water Measured Under the Lower Tropospheric Conditions  

NASA Astrophysics Data System (ADS)

Nucleation is a gas to particle conversion process in which solid or liquid aerosol particles form directly from the gas phase species and thus is an important step in the chain of reactions that lead to cloud formation. However, the nucleation mechanisms are poorly understood. There are presently large discrepancies amongst the measured nucleation rates by different laboratory studies; nucleation rates taken under the atmospheric conditions are very rare. We perform laboratory experiments of aerosol nucleation under the atmospheric conditions with a novel chemical ionization mass spectrometer (CIMS) and nano-particle differential mobility analyzer (Nano-DMA) and water condensation nuclei counter (WCPC). Our CIMS measures low concentrations of sulfuric acid (down to 1e-6 cm-3). Measurements of Nano-DMA and WCPC provide aerosol nucleation rates. An atmospheric pressure flow reactor is used to photochemically produce sulfuric acid particles from the reactions of OH and sulfur dioxide. Sulfur dioxide is also detected by CIMS. OH forms from the photo-dissociation of water vapor using a mercury lamp and the OH concentrations are calculated based on the known photochemical reaction rates and measured photon fluxes. This OH generation method also allows for sulfuric acid calibration. We will present nucleation rates of sulfuric acid and water as a function of sulfuric acid concentration, RH and temperature under the conditions of lower troposphere. We also compare these results with those by Berndt et al. (2005) that show relatively high concentrations of sulfuric acid and water nucleation rates comparable to the atmospheric observations.

Young, L.; Benson, D. R.; Quinn, K.; Lee, S.

2006-12-01

146

Gas phase and surface reactions in subatmospheric chemical vapor deposition of tetraethylorthosilicate-ozone  

SciTech Connect

A new physical-chemical model, which applies over a wide range of operating pressures, describes the gas phase and surface reactions in subatmospheric chemical vapor deposition of silicon dioxide for producing inter-layer dielectrics in a cold-wall reactor. Tetraethylorthosilicate (TEOS) reacts in the gas phase to form an intermediate which is adsorbed and reacts on the surface to produce a silicon dioxide film. The results compare favorably with experimental data over a pressure range of 100{endash}600 Torr and a temperature range of 370{endash}500{degree}C. The concentration distributions of TEOS, intermediate and ozone in the gas phase and their ratios at the surface of the wafer are determined to study gas phase nucleation and the relationship between composition distributions and film quality. Previous models based on low pressure data in the range of 30{endash}90 Torr need to be modified to predict accurately the rates of deposition from 100 Torr to atmospheric pressure. Gas phase reactions cause the maximum in the deposition rates to shift to higher pressures at lower deposition temperatures, both in the model and experiments. At higher pressures, particulates are formed by the gas phase reactions which must be included to represent properly the chemical dynamics of the process. The deposition rate increases up to an asymptotic value as the TEOS flow rate is increased; above this level no further increase in growth rate occurs. This is a consequence of the basic mechanism of the surface reaction which predicts the asymptotic behavior observed. {copyright} {ital 1997 American Vacuum Society.}

Gill, W.N.; Ganguli, S. [Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (United States)

1997-07-01

147

Vaccum Gas Tungsten Arc Welding, phase 1  

NASA Astrophysics Data System (ADS)

This two year program will investigate Vacuum Gas Tungsten Arc Welding (VGTAW) as a method to modify or improve the weldability of normally difficult-to-weld materials. VGTAW appears to offer a significant improvement in weldability because of the clean environment and lower heat input needed. The overall objective of the program is to develop the VGTAW technology and implement it into a manufacturing environment that will result in lower cost, better quality and higher reliability aerospace components for the space shuttle and other NASA space systems. Phase 1 of this program was aimed at demonstrating the process's ability to weld normally difficult-to-weld materials. Phase 2 will focus on further evaluation, a hardware demonstration and a plan to implement VGTAW technology into a manufacturing environment. During Phase 1, the following tasks were performed: (1) Task 11000 Facility Modification - an existing vacuum chamber was modified and adapted to a GTAW power supply; (2) Task 12000 Materials Selection - four difficult-to-weld materials typically used in the construction of aerospace hardware were chosen for study; (3) Task 13000 VGTAW Experiments - welding experiments were conducted under vacuum using the hollow tungsten electrode and evaluation. As a result of this effort, two materials, NARloy Z and Incoloy 903, were downselected for further characterization in Phase 2; and (4) Task 13100 Aluminum-Lithium Weld Studies - this task was added to the original work statement to investigate the effects of vacuum welding and weld pool vibration on aluminum-lithium alloys.

Weeks, J. L.; Krotz, P. D.; Todd, D. T.; Liaw, Y. K.

1995-03-01

148

Hydrogen atoms and gas phase chemical dynamics  

SciTech Connect

This thesis describes the applications of Doppler spectroscopy of hydrogen atoms to gas phase chemical dynamics. Hydrogen atoms are generated in the electronic ground state (1s) either from photodissociation or from chemical reactions. They are then excited to the 2p state by 121.6 nm light. The fluorescence intensity (2p [yields] 1s) of the H atoms is monitored by a photomultiplier tube. The measurement of the H atom velocity helps us to follow chemical reactions in the gas phase occurring in a very short time. There are three experimental works described here. The first one shows how the Walden inversion, previously observed indirectly by organic chemists, is followed in the reaction of H atoms with CD[sub 4]. The reactions of electronically excited Xe and Kr atoms with H[sub 2] is illustrated in the next work. The indirect evidence for the formation of XeD and KrD is presented. In the third work, the photodissociation dynamics of small molecules like HCl, HCN and C[sub 2]H[sub 2] is discussed.

Chattopadhyay, A.

1992-01-01

149

Gas-phase thermochemistry of chloropyridines  

NASA Astrophysics Data System (ADS)

The gas-phase standard molar enthalpy of formation of the 2,3,5-trichloropyridine compound was derived from the enthalpies of combustion of the crystalline solid measured by rotating-bomb calorimetry and its enthalpy of sublimation obtained by Calvet microcalorimetry at T = 298.15 K. The standard enthalpies of formation for this compound and for the other chlorosubstituted pyridines were determined by DFT calculations. The experimental enthalpy of formation of 2,3,5-trichloropyridine is (65.8 ± 2.3) kJ mol -1, in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) value. The affinity of pyridine to some metal cations was also calculated at the same DFT level of theory and compared with experimental data.

Gomes, José R. B.; Amaral, Luísa M. P. F.; Ribeiro da Silva, Manuel A. V.

2005-04-01

150

Gas phase hyper-Rayleigh scattering measurements  

NASA Astrophysics Data System (ADS)

Measurements of hyper-Rayleigh scattering intensities and polarization ratios are presented for nine small molecules in the gas phase [CH4, CF4, CCl4, N2O, NH3, D2O, SO2, CF2Cl2, and (CH3)2CO]. In four cases [CH4, CF4, CCl4, and N2O] all molecular hyperpolarizability tensor components can be determined from the measurements. The results of this experiment are compared with the results of previous ab initio calculations, finding discrepancies up to 60%. Including vibrational contributions decreases the discrepancies for CH4 and CF4 and increases them for CCl4, D2O, and NH3.

Shelton, David P.

2012-07-01

151

Femtosecond lasers in gas phase chemistry.  

PubMed

This critical review is intended to provide an overview of the state-of-the-art in femtosecond laser technology and recent applications in ultrafast gas phase chemical dynamics. Although "femtochemistry" is not a new subject, there have been some tremendous advances in experimental techniques during the last few years. Time-resolved photoelectron spectroscopy and ultrafast electron diffraction have enabled us to observe molecular dynamics through a wider window. Attosecond laser sources, which have so far only been exploited in atomic physics, have the potential to probe chemical dynamics on an even faster timescale and observe the motions of electrons. Huge progress in pulse shaping and pulse characterisation methodology is paving the way for exciting new advances in the field of coherent control. PMID:16239996

Carley, R E; Heesel, E; Fielding, H H

2005-09-29

152

Transferring pharmaceuticals into the gas phase  

NASA Astrophysics Data System (ADS)

The dissolution of molecules of biological interest in supercritical carbon dioxide is investigated using pulsed molecular beam mass spectrometry. Due to the mild processing temperatures of most supercritical fluids, their adiabatic expansion into vacuum permits to transfer even thermally very sensitive substances into the gas phase, which is particularly attractive for pharmaceutical and biomedical applications. In addition, supercritical CO2constitutes a chemically inert solvent that is compatible with hydrocarbon-free ultrahigh vacuum conditions. Here, we report on the dissolution and pulsed supersonic jet expansion of caffeine (C8H10N4O2), the provitamin menadione (C11H8O2), and the amino acid derivative l-phenylalanine tert-butyl ester hydrochloride (C6H5CH2CH(NH2)COOC(CH3)3[dot operator]HCl), into vacuum. An on-axis residual gas analyzer is used to monitor the relative amounts of solute and solvent in the molecular beam as a function of solvent densityE The excellent selectivity and sensitivity provided by mass spectrometry permits to probe even trace amounts of solutes. The strong density variation of CO2 close to the critical point results in a pronounced pressure dependence of the relative ion currents of solute and solvent molecules, reflecting a substantial change in solubility.

Christen, Wolfgang; Krause, Tim; Rademann, Klaus

2008-11-01

153

Gas phase protonation of trifluoromethyl sulfur pentafluoride.  

PubMed

The gas phase protonation of SF5CF3, a potent new greenhouse gas recently discovered in stratospheric air samples, was studied by the joint application of mass spectrometric and ab initio theoretical methods. The reaction is essentially dissociative leading to the formation of HF, CF4 and SF3+ as main fragmentation products. Consistent with collisionally activated dissociation (CAD) mass spectrometric results, theoretical calculations identified the loosely bounded ion-molecule complex [HF-SF4 CF3], I, as the most stable isomer on the [SF5CF3]H+ potential energy surface. The proton affinity of SF5CF3 estimated from FT-ICR 'bracketing' experiments was found to be 152.5 +/- 3 kcal mol(-1) which agrees with the values obtained from theoretical calculations at B3LYP and CCSD(T) levels of theory, 154.0 +/- 3 and 153.4 +/- 3 kcal mol(-1), respectively. These results suggest that the basicity of SF5CF3 is higher than that of atmospheric cations such as H2O+; they need to be considered when evaluating the lifetime of SF5CF3 since it can be destroyed by proton transfer reactions. PMID:19791331

Pepi, Federico; Ricci, Andreina; Di Stefano, Marco; Rosi, Marzio

2005-03-21

154

Gas: A Neglected Phase in Remediation of Metals and Radionuclides  

SciTech Connect

The gas phase is generally ignored in remediation of metals and radionuclides because it is assumed that there is no efficient way to exploit it. In the literal sense, all remediations involve the gas phase because this phase is linked to the liquid and solid phases by vapor pressure and thermodynamic relationships. Remediation methods that specifically use the gas phase as a central feature have primarily targeted volatile organic contaminants, not metals and radionuclides. Unlike many organic contaminants, the vapor pressure and Henry's Law constants of metals and radionuclides are not generally conducive to direct air stripping of dissolved contaminants. Nevertheless, the gas phase can play an important role in remediation of inorganic contaminants and provide opportunities for efficient, cost effective remediation. The objective here is to explore ways in which manipulation of the gas phase can be used to facilitate remediation of metals and radionuclides.

Denham, Miles E.; Looney, Brian B

2005-09-28

155

Small-volume nucleation  

NASA Astrophysics Data System (ADS)

The nucleation mechanisms behind crystallized products remain mysterious. In this communication, we describe experiments performed using small volumes, microdroplets, to control nucleation and thus product properties. The effect of small-volume systems on nucleation is discussed.

Hammadi, Zoubida; Candoni, Nadine; Grossier, Romain; Ildefonso, Manuel; Morin, Roger; Veesler, Stéphane

2013-02-01

156

Recent progress in understanding particle nucleation and growth  

PubMed Central

In the past half decade, several new tools have become available for investigating particle nucleation and growth. A number of joint field and laboratory studies exploiting some of these new measurement capabilities will be described and new insights shared. the ability to measure OH, SO2, H2SO4 and aerosol number and size distributions has made possible a comparison between H2SO4 production and loss onto particles in continental air masses. In regions remote from urban emissions, agreement is typically quite good. In contrast, joint field measurements of nucleation precursors such as gas phase H2SO4 and ultrafine particles suggest that classical bimolecular nucleation theory may not properly describe the tropospheric nucleation process. An alternative mechanism, possibly involving ammonia as a stabilizing agent for H2SO4/H2O molecular clusters is discussed. Finally, ultrafine particle measurements are shown to offer new opportunities for studying particle growth rates. Preliminary results suggest that in a remote continental air mass, gas phase H2SO4 uptake is far too slow to explain observed growth rates.

Eisele, F. L.; McMurry, P. H.

1997-01-01

157

Studies on Pressure Response of Gas Bubbles Contributions of Condensed Droplets in Bubbles Generated by a Uniform Nucleation.  

National Technical Information Service (NTIS)

The response of a tiny gas bubble under reduced pressure is investigated in its relation to cavitation. Equations of motion are formulated for gas mixtures inside the bubble and numerical calculations performed for several examples. The conclusions are as...

Y. Matsumoto

1988-01-01

158

Gas-Phase Spectroscopy of Biomolecular Building Blocks  

Microsoft Academic Search

Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from

Mattanjah S. de Vries; Pavel Hobza

2007-01-01

159

How Ubiquitin Unfolds after Transfer into the Gas Phase  

NASA Astrophysics Data System (ADS)

The structural evolution of ubiquitin after transfer into the gas phase was studied by electron capture dissociation. Site-specific fragment yields show that ubiquitin's solution fold is overall unstable in the gas phase, but unfolding caused by loss of solvent is slowest in regions stabilized by salt bridges.

Skinner, Owen S.; McLafferty, Fred W.; Breuker, Kathrin

2012-06-01

160

Gas Phase Ignition of a Hypergolic Propellant System  

Microsoft Academic Search

A method of measuring ignition delay of a gas phase hypergolic propellant system in quasi steady flow and low pressure conditions is described. The effects of ambient pressure, flow parameters and propellant temperature on the gas phase ignition delay of the system nitrogen tetroxide-UDMH are investigated. An empirical equation relating the ignition delay to ambient pressure and temperature is obtained

HIROSHI MIYAJIMA; HIROSHI SAKAMOTO

1971-01-01

161

Gas-phase basicity of 2-furaldehyde.  

PubMed

2-Furaldehyde (2-FA), also known as furfural or 2-furancarboxaldehyde, is an heterocyclic aldehyde that can be obtained from the thermal dehydration of pentose monosaccharides. This molecule can be considered as an important sustainable intermediate for the preparation of a great variety of chemicals, pharmaceuticals and furan-based polymers. Despite the great importance of this molecule, its gas-phase basicity (GB) has never been measured. In this work, the GB of 2-FA was determined by the extended Cooks's kinetic method from electrospray ionization triple quadrupole tandem mass spectrometric experiments along with theoretical calculations. As expected, computational results identify the aldehydic oxygen atom of 2-FA as the preferred protonation site. The geometries of O-O-cis and O-O-trans 2-FA and of their six different protomers were calculated at the B3LYP/aug-TZV(d,p) level of theory; proton affinity (PA) values were also calculated at the G3(MP2, CCSD(T)) level of theory. The experimental PA was estimated to be 847.9 ± 3.8 kJ mol(-1), the protonation entropy 115.1 ± 5.03 J mol(-1) K(-1) and the GB 813.6 ± 4.08 kJ mol(-1) at 298 K. From the PA value, a ?H°(f) of 533.0 ± 12.4 kJ mol(-1) for protonated 2-FA was derived. PMID:23147827

Ricci, Andreina; Piccolella, Simona; Pepi, Federico; Patsilinakos, Alexandros; Ragno, Rino; Garzoli, Stefania; Giacomello, Pierluigi

2012-11-01

162

Intramolecular photoelectron diffraction in the gas phase  

NASA Astrophysics Data System (ADS)

We report unambiguous experimental and theoretical evidence of intramolecular photoelectron diffraction in the collective vibrational excitation that accompanies high-energy photoionization of gas-phase CF4, BF3, and CH4 from the 1s orbital of the central atom. We show that the ratios between vibrationally resolved photoionization cross sections (v-ratios) exhibit pronounced oscillations as a function of photon energy, which is the fingerprint of electron diffraction by the surrounding atomic centers. This interpretation is supported by the excellent agreement between first-principles static-exchange and time-dependent density functional theory calculations and high resolution measurements, as well as by qualitative agreement at high energies with a model in which atomic displacements are treated to first order of perturbation theory. The latter model allows us to rationalize the results for all the v-ratios in terms of a generalized v-ratio, which contains information on the structure of the above three molecules and the corresponding molecular cations. A fit of the measured v-ratios to a simple formula based on this model suggests that the method could be used to obtain structural information of both neutral and ionic molecular species.

Ueda, K.; Miron, C.; Plésiat, E.; Argenti, L.; Patanen, M.; Kooser, K.; Ayuso, D.; Mondal, S.; Kimura, M.; Sakai, K.; Travnikova, O.; Palacios, A.; Decleva, P.; Kukk, E.; Martín, F.

2013-09-01

163

Atmospheric ion-induced nucleation of sulfuric acid and water  

NASA Astrophysics Data System (ADS)

Field studies show that gas phase nucleation is an important source of new particles in the Earth's atmosphere. However, the mechanism of new particle formation is not known. The predictions of current atmospheric nucleation models are highly uncertain because the models are based on estimates for the thermodynamics of cluster growth. We have measured the thermodynamics for the growth and evaporation of small cluster ions containing H2SO4 and H2O, and incorporated these data into a kinetic aerosol model to yield quantitative predictions of the rate of ion-induced nucleation for atmospheric conditions. The model predicts that the binary negative ion H2SO4/H2O mechanism is an efficient source of new particles in the middle and upper troposphere. The ion-induced HSO4-/H2SO4/H2O mechanism does explain nucleation events observed in the remote middle troposphere, but does not generally predict the nucleation events observed in the boundary layer.

Lovejoy, E. R.; Curtius, J.; Froyd, K. D.

2004-04-01

164

Borate Ions in the Gas Phase, Fast-Atom Bombardment of Condensed-Phase Borates.  

National Technical Information Service (NTIS)

The intractable properties of crystalline and vitreous, condensed-phase boron-oxygen compounds have always hindered the generation and analysis of gas-phase boron-oxygen species. Recently, structures of gas-phase boron oxide cations, generated by fast-ato...

R. J. Doyle

1988-01-01

165

Crystal nucleation of colloidal hard dumbbells  

NASA Astrophysics Data System (ADS)

Using computer simulations, we investigate the homogeneous crystal nucleation in suspensions of colloidal hard dumbbells. The free energy barriers are determined by Monte Carlo simulations using the umbrella sampling technique. We calculate the nucleation rates for the plastic crystal and the aperiodic crystal phase using the kinetic prefactor as determined from event driven molecular dynamics simulations. We find good agreement with the nucleation rates determined from spontaneous nucleation events observed in event driven molecular dynamics simulations within error bars of one order of magnitude. We study the effect of aspect ratio of the dumbbells on the nucleation of plastic and aperiodic crystal phases, and we also determine the structure of the critical nuclei. Moreover, we find that the nucleation of the aligned close-packed crystal structure is strongly suppressed by a high free energy barrier at low supersaturations and slow dynamics at high supersaturations.

Ni, Ran; Dijkstra, Marjolein

2011-01-01

166

A critical reexamination of the fundamentals of classical nucleation theory  

NASA Astrophysics Data System (ADS)

This thesis examines the basic presumptions behind the classic nucleation theory (CNT). Nucleation is the process by which clusters of a stable phase form in a metastable phase. CNT depicts the process as the clusters grow and shrink through the attachment and detachment reactions with monomers ( monomer approximation). It assumes the metastable phase obeys the laws of fluctuation thermodynamics (constrained equilibrium hypothesis ) and partitions a cluster's free energy into a bulk term and a surface term having constant energy density equal to their macroscopic values ( capillarity approximation). With the aid of molecular dynamics (MD) computations that simulate the nucleation of supersaturated Lennard-Jones gas, we discovered that the constrained equilibrium hypothesis and the capillarity approximation do not hold, while the monomer approximation is acceptable due to the breakdown of the detailed balance. We also studied the mechanism behind the binary to unary transition problem (the nucleation rate goes to zero when one component of a binary system diminishes) and provided a simple formula to predict the rate correctly.

Zhuo, Lin

167

Thermodynamic and kinetic consistency of calculated binary nucleation rates  

SciTech Connect

To establish the accuracy and applicability of analytical expressions for the steady state rate of binary nucleation, we numerically solved the birth-death equations for the vapor-to-liquid transition. These calculations were performed using rate coefficients that are consistent with the principle of detailed balance and a new self-consistent form of the equilibrium distribution function for binary cluster concentrations. We found that the customary saddle point and growth path approximations are almost always valid and can fail only if the nucleating solution phase is significantly nonideal. For example, problems can arise when the vapor composition puts the system on the verge of partial liquid phase miscibility. When this occurs for comparable monomer impingement rates, nucleation still occurs through the saddle point, but the usual quadratic expansion for the cluster free energy is inadequate. When the two impingement rates differ significantly, however, the major particle flux may bypass the saddle point and cross a low ridge on the free energy surface. The dependence of the saddle point location on the gas phase composition is also important in initiating or terminating ridge crossing nucleation.

Wilemski, G. [Lawrence Livermore National Lab., CA (United States); Wyslouzil, B.E. [Worcester Polytechnic Institute, MA (United States)

1996-04-02

168

Real-time spectroscopic ellipsometry studies of the nucleation and grain growth processes in microcrystalline silicon thin films  

Microsoft Academic Search

Real-time spectroscopic ellipsometry (SE) has been applied to investigate the nucleation and grain growth processes in microcrystalline silicon (muc-Si:H) thin films deposited by a conventional plasma-enhanced chemical vapor deposition using hydrogen dilution of silane source gas. Real-time SE results revealed the muc-Si:H nucleation from hydrogenated amorphous silicon (a-Si:H) phase, followed by the coalescence of isolated muc-Si:H grains exposed on growing

Hiroyuki Fujiwara; Michio Kondo; Akihisa Matsuda

2001-01-01

169

Gas-phase selective etching of native oxide  

Microsoft Academic Search

Gas-phase selective etching of native oxide film formed on a silicon surface is an essential requirement for ULSI process technologies. Ultraclear anhydrous hydrogen fluoride (AHF) gas and a corrosion-free system were developed for this etching process. The reaction mechanism of silicon oxide film with moistureless HF was investigated, and selective etching conditions were developed. The gas-phase selective etching of native

NOBUHIRO MIKI; HIROHISA KIKUYAMA; ICHIROH KAWANABE; MASAYUKI MIYASHITA; TADAHIRO OHMI

1990-01-01

170

Ab initio study of gas-phase sulphuric acid hydrates containing 1 to 3 water molecules  

NASA Astrophysics Data System (ADS)

Sulphuric acid has a tendency to form hydrates, small clusters containing a few water molecules, in the gas phase. Hydrate formation has a stabilising effect on the vapour as the pressure of sulphuric acid drops (relative to unhydrated vapor), decreasing the nucleation rate. In classical nucleation theories the hydration energies and the hydrate distribution are predicted assuming that hydrates can be described as liquid droplets having thermodynamic properties of bulk liquid. To obtain a better understanding of the structures and formation energies of the smallest clusters, we have performed ab initio density functional calculations of the mono-,di-, and trihydrates. The hydrogen bonds between the molecules are found to be strong. The more water molecules the hydrate contains, the clearer ring-like structure is formed. Comparison to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the smallest clusters too well. The energy barrier for proton transfer reaction H2SO4.H2O -->HSO4-.H3O+ for mono- and dihydrate is high, and protonisation is unlikely to occur, but in trihydrate the protonisation has almost occurred and the barrier is very low. We also studied the singly protonised monohydrate, and found that while sulphuric acid forms H bonds with the OH parts, the hydrogen sulphate ion tends to bind with the O (S=O) part, and the second proton stays tightly in the ion.

Arstila, Hanna; Laasonen, Kari; Laaksonen, Ari

1998-01-01

171

Gas-phase nitronium ion affinities.  

PubMed Central

Evaluation of nitronium ion-transfer equilibria, L1NO2+ + L2 = L2NO2+ + L1 (where L1 and L2 are ligands 1 and 2, respectively) by Fourier-transform ion cyclotron resonance mass spectrometry and application of the kinetic method, based on the metastable fragmentation of L1(NO2+)L2 nitronium ion-bound dimers led to a scale of relative gas-phase nitronium ion affinities. This scale, calibrated to a recent literature value for the NO2+ affinity of water, led for 18 ligands, including methanol, ammonia, representative ketones, nitriles, and nitroalkanes, to absolute NO2+ affinities, that fit a reasonably linear general correlation when plotted vs. the corresponding proton affinities (PAs). The slope of the plot depends to a certain extent on the specific nature of the ligands and, hence, the correlations between the NO2+ affinities, and the PAs of a given class of compounds display a better linearity than the general correlation and may afford a useful tool for predicting the NO2+ affinity of a molecule based on its PA. The NO2+ binding energies are considerably lower than the corresponding PAs and well below the binding energies of related polyatomic cations, such as NO+, a trend consistent with the available theoretical results on the structure and the stability of simple NO2+ complexes. The present study reports an example of extension of the kinetic method to dimers, such as L1(NO2+)L2, bound by polyatomic ions, which may considerably widen its scope. Finally, measurement of the NO2+ affinity of ammonia allowed evaluation of the otherwise inaccessible PA of the amino group of nitramide and, hence, direct experimental verification of previous theoretical estimates.

Cacace, F; de Petris, G; Pepi, F; Angelelli, F

1995-01-01

172

1,2-propanediol and 1,3-propanediol homogeneous nucleation rates and phase transitions in the new phase critical embryos  

Microsoft Academic Search

Nucleation rates of supersaturated vapors near the conditions for the critical line for each pair of 1,2- or 1,3-propanediol and carbon dioxide or sulfur hexafluoride at total system pressures of P=0.10, 0.20, and 0.30 MPa have been measured in a flow diffusion chamber. Critical parameters, i.e., temperatures and pressures, of the binary systems versus compositions were semiempirically evaluated. It was

M. P. Anisimov; J. A. Koropchak; A. G. Nasibulin; L. V. Timoshina

2000-01-01

173

A model prediction of the yield of cloud condensation nuclei from coastal nucleation events  

Microsoft Academic Search

The formation and evolution of new particles during coastal nucleation events are examined using the aerosol dynamic and gas-phase chemistry model AEROFOR2. Coastal regions are known to be a strong source of natural aerosol particles and are also strong sources of biogenic vapors which can condense onto aerosol particles, thus resulting in particle growth. A number of model simulations were

Liisa Pirjola; Colin D. O'Dowd; Markku Kulmala

2002-01-01

174

Nucleation and Coalescence of Indium Rich InGaN Layers on Nitridated Sapphire in Metal--Organic Vapor Phase Epitaxy  

NASA Astrophysics Data System (ADS)

We report on the evolution of N-polar wurtzite (000\\bar{1}) InGaN layers with high indium content grown directly on nitridated (0001) sapphire by metal--organic vapor phase epitaxy. In-situ ellipsometry showed an initial growth delay of 5--8 nm for layers with more than 45% indium content. Atomic force microscopy showed small dense islands which reached their final size after 5--8 nm deposition. The roughness saturated only after 15 nm, and the width of the X-ray diffraction (00.2) reflex showed a similar behavior. Therefore, N-polar InGaN grows via nucleation (d < 10 nm) followed by coalescence (d < 20 nm). With less than 40% indium the coalescence is much slower, very similar to Ga-polar (0001) GaN despite N-polarity of the InN layers. This indicates towards a surface transition between 40 and 50% indium content.

Dinh, Duc V.; Solopow, Sergej; Pristovsek, Markus; Kneissl, Michael

2013-08-01

175

Translation-rotation correction factor in the theory of homogeneous nucleation  

NASA Astrophysics Data System (ADS)

An analytical formula for the correction factor which is to multiply the classical expression for the nucleation rate to account the translation and rotation of the critical nucleus is proposed. The formula is based on the Reiss approach considering the contribution from the clusters translational degrees of freedom, Frenkel's kinetic theory of liquids, and Kusaka's theory. Using this formula we determined the correction factor for argon vapor-to-liquid phase nucleation for the temperature range 80-110 K. These evaluations are in a good agreement with the correction factor calculated numerically by Kusaka (2006). Basing on the Gibbs theory of capillarity it is also shown that for the case of ideal gas-to-liquid nucleation the exponent in the classical formula for the rate of nucleation is strictly equal to the reversible work of drop formation.

Vosel, S. V.; Onischuk, A. A.; Purtov, P. A.

2009-11-01

176

Nonisothermal nucleation in the CuCl solid solution in glass: Formation of two distributions of nanoparticles of a new phase in the solid solution with a negative jump of the nucleation temperature  

NASA Astrophysics Data System (ADS)

The processes of nonisothermal nucleation in the CuCl solid solution in glass with a negative jump of the nucleation temperature have been investigated using optical spectroscopy and exciton-thermal analysis. Two distributions of CuCl nanoparticles differing in the average radii R (2.3 and 14.0 nm) have been obtained for the temperature drop from T 1 = 700°C to T 2 = 500°C. The formation of two distributions of CuCl particles has been numerically simulated and the agreement between the experimental and calculated radius distributions of CuCl particles has been obtained.

Leiman, V. I.; Valov, P. M.; Maksimov, V. M.; Derkacheva, O. Yu.; Markov, E. S.

2013-06-01

177

Dissolution of two-phase microsystems: Gas and liquid microparticle dissolution and dehydration of biomaterials  

NASA Astrophysics Data System (ADS)

A main focus of this research is to develop techniques to study the dissolution process of two-phase microsystems on a single microparticle basis. This dissertation introduces a systematic approach to investigate the formation of microparticles to fulfill the need for rational design of microspheres for a range of applications. This novel method is based on the micropipet manipulation technique and can essentially test any system, where the continuous phase is a liquid and the dispersed phase is practically any phase, a gas (foam), a liquid (emulsion), or a solid (suspension). It is possible to study single microparticle volumes in the picoliter to nanoliter scale, which is on the same size-scale as particles created in bulk suspensions, microsphere processes, and applications. The ability to create, isolate, observe, and manipulate individual gas, liquid or solid microparticles in a well-defined and controlled liquid environment was found to be ideal to study gas microbubbles and microparticles, liquid microdroplets, and the dehydration of dissolved solutes. Subsequently, one can directly measure the dissolution rate and, when a solute is present, calculate its concentration during the dissolution process. Microbubble or microdroplet dissolution in a second phase is driven by two independent factors, a concentration gradient (undersaturation of the dispersed phase in the continuous phase) and a pressure gradient (due to the Laplace-overpressure inside the microparticle created by the surface tension). Experimentally, each of these driving forces can be independently tested. Both the gas microparticle and pure liquid microdroplet dissolution can be predicted by a simple theory based on the diffusion coefficient and solubility limit of the dispersed phase in the continuous phase. The dehydration of a salt ion solution microdroplet results in the nucleation and growth of a crystal, while the dehydration of proteins leads to glassification of the protein. The water remaining in the glassified protein microsphere is on the order of a water monolayer surrounding each protein molecule. Both observation and measurement of dehydration within a single microdroplet is the basis to understanding microparticle formation for use in drug delivery systems and biomolecule preservation.

Duncan, Phillip Brent

178

Amplification of Sound by Gas Phase Reactions.  

National Technical Information Service (NTIS)

A four year study of sound propagation in chemically reacting mixtures has led to experimental observation of sound amplification. Photo-initiated C12-H2-inert gas reactions provided the energy for the amplification observed. Amplification experiments wer...

H. E. Bass R. M. Detsch

1983-01-01

179

Absorption of Gas Phase Contaminant Mixtures.  

National Technical Information Service (NTIS)

This project seeks to study the effect of multiple gas-vapor mixtures on the activated carbon adsorption breakthrough time and pattern. Activated carbon cartridges used for respiratory protection are tested in most cases for a single component at relative...

C. Lungu

2004-01-01

180

Gas-Phase Spectroscopy of Biomolecular Building Blocks  

NASA Astrophysics Data System (ADS)

Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

de Vries, Mattanjah S.; Hobza, Pavel

2007-05-01

181

Gas-Phase Aldol Condensation Over Tin on Silica Catalysts.  

National Technical Information Service (NTIS)

Contents: Aldol condensation; Catalyst preparation and composition; Catalyst performance; Kinetics; On the nature of the active site for the aldol condensation over tin on silica catalysts; Gas-phase aldol condensation of n-butanal over tin on silica cata...

J. Venselaar

1980-01-01

182

Development and validation of a portable gas phase standard ...  

Treesearch

MOCCS was validated using three different comparisons: (1) TOC of high ... (2) a gas-phase benzene standard was generated using a permeation source and ... agreement (<4% relative difference), and (3) total carbon measurement of 4 ...

183

CHEMKIN2. General Gas-Phase Chemical Kinetics  

Microsoft Academic Search

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary,

Rupley

1992-01-01

184

The gas phase oxidation of elemental mercury by ozone  

Microsoft Academic Search

The gas phase reaction between elemental mercury (Hg0) and ozone (03) has been studied in sunlight, in darkness, at different temperatures, and different surface-to-volume (s\\/v) ratios. At 03 concentrations above 20 ppm, a loss of Hg0 and a simultaneous formation of oxidized mercury (Hg(II)) was observed. The results suggest a partly heterogeneous reaction, with a gas phase rate constant of

B. Hall

1995-01-01

185

Gas-phase purification of single-wall carbon nanotubes  

Microsoft Academic Search

A gas-phase purification method for raw nanotube material has been developed which incorporates a chlorine, water, and hydrogen chloride gas mixture to remove unwanted carbon. The evolved gases can be easily monitored by infrared spectroscopy to follow the cleaning process. The quality of the final material was verified by SEM (scanning electron microscopy), TGA (thermogravimetric analysis), and UV-vis (ultraviolet and

John L. Zimmerman; Robert Kelley Bradley; Chad B. Huffman; Robert H. Hauge; John L. Margrave

2000-01-01

186

Carbon Dioxide Separation from Flue Gas by Phase Enhanced Absorption  

Microsoft Academic Search

A new process, phase enhanced absorption, was invented. The method is carried out in an absorber, where a liquid carrier (aqueous solution), an organic mixture (or organic compound), and a gas mixture containing a gas to be absorbed are introduced from an inlet. Since the organic mixture is immiscible or at least partially immiscible with the liquid carrier, the organic

Tim Fout

2007-01-01

187

Substrate orientation effects on the nucleation and growth of the Mn+1AXn phase Ti2AlC  

Microsoft Academic Search

The Mn+1AXn (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third ``A-group'' element. The effect of substrate orientation on the growth of Ti2AlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (1010), and (1102) crystallographic planes. Characterization of these samples

Mark D. Tucker; Mathew C. Guenette; Johanna Rosén; Marcela M. M. Bilek; David R. McKenzie

2011-01-01

188

Substrate orientation effects on the nucleation and growth of the Mn+1AXn phase Ti2AlC  

Microsoft Academic Search

The Mn+1AXn (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third “A-group” element. The effect of substrate orientation on the growth of Ti2AlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (1010), and (1102) crystallographic planes. Characterization of these samples

Mark D. Tucker; Mathew C. Guenette; Johanna Rose´n; Marcela M. M. Bilek; David R. McKenzie

2011-01-01

189

Nucleation of electroactive ?-phase poly(vinilidene fluoride) with CoFe2O4 and NiFe2O4 nanofillers: a new method for the preparation of multiferroic nanocomposites  

NASA Astrophysics Data System (ADS)

Multiferroic and magnetoelectric materials show enormous potential for technological developments. Multiferroic composites are more attractive for applications due to their enhanced properties with respect to single-phase multiferroic materials. In this paper we report on the nucleation of the electroactive ?-phase of poly(vinylidene fluoride), PVDF, by the addition of CoFe2O4 and NiFe2O4 nanoparticles in order to prepare poly(vinylidene fluoride)/ferrite nanocomposite for multiferroic and magnetoelectric applications,. The dispersed ferrite nanofiller particles strongly enhance the nucleation of the ?-phase of the polymer matrix. In this way, magnetoelectric polymer nanocomposites can be processed avoiding the usual ?- to ?-phase transformation by stretching of the polymer matrix.

Martins, P.; Costa, C. M.; Lanceros-Mendez, S.

2011-04-01

190

Chemistry with weakly-coordinating fluorinated alkoxyaluminate anions: Gas phase cations in condensed phases?  

Microsoft Academic Search

Many very simple and fundamentally important cations were initially observed in the gas phase, e.g. inside a mass spectrometer or even in space. The question arises, how to stabilize and study these gas phase cations in condensed phases. In this review, our work on the use of weakly-coordinating anions (WCAs) of type [Al(ORF)4]? and [(RFO)3Al–F–Al(ORF)3]? to investigate structural and chemical

Ingo Krossing; Andreas Reisinger

2006-01-01

191

High resolution ion mobility measurements for gas phase proteins: correlation between solution phase and gas phase conformations  

NASA Astrophysics Data System (ADS)

Our high resolution ion mobility apparatus has been modified by attaching an electrospray source to perform measurements for biological molecules. While the greater resolving power permits the resolution of more conformations for BPTI and cytochrome c, the resolved features are generally much broader than expected for a single rigid conformation. A major advantage of the new experimental configuration is the much gentler introduction of ions into the drift tube, so that the observed gas phase conformations appear to more closely reflect those present in solution. For example, it is possible to distinguish between the native state of cytochrome c and the methanol-denatured form on the basis of the ion mobility measurements; the mass spectra alone are not sensitive enough to detect this change. Thus this approach may provide a quick and sensitive tool for probing the solution phase conformations of biological molecules.

Hudgins, Robert R.; Woenckhaus, Jürgen; Jarrold, Martin F.

1997-11-01

192

X-Ray Absorption Spectroscopy of Gas Phase Metal Clusters.  

National Technical Information Service (NTIS)

The production and control of vapor phase metal clusters in sizes ranging from one to thousands of atoms is discussed. Electron yield detection has a very high sensitivity to species in the gas phases. These events suggested the possibility of a combined ...

E. C. Marques D. S. Olson D. R. Sandstron F. W. Lytle R. B. Greegor

1986-01-01

193

Nucleation on active sites: evolution of size distribution  

Microsoft Academic Search

Transient 3D substrate nucleation and growth from the vapor phase on the active sites is studied. The set of kinetic equations describing the formation of nuclei, including the depletion of active sites during nucleation, has been solved numerically within standard nucleation theory to obtain the time-dependent size distribution function of nuclei. Knowing the size distribution function, the basic characteristics of

Z Kož??šek; P Demo; K Sato

2000-01-01

194

Substrate orientation effects on the nucleation and growth of the Mn+1AXn phase Ti2AlC  

NASA Astrophysics Data System (ADS)

The Mn+1AXn (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third ``A-group'' element. The effect of substrate orientation on the growth of Ti2AlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (1010), and (1102) crystallographic planes. Characterization of these samples was by x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. On the (1010) substrate, tilted (1018) growth of Ti2AlC was found, such that the TiC octahedra of the MAX phase structure have the same orientation as a spontaneously formed epitaxial TiC sublayer, preserving the typical TiC-Ti2AlC epitaxial relationship and confirming the importance of this relationship in determining MAX phase film orientation. An additional component of Ti2AlC with tilted fiber texture was observed in this sample; tilted fiber texture, or axiotaxy, has not previously been seen in MAX phase films.

Tucker, Mark D.; Persson, Per O. A.?.; Guenette, Mathew C.; Rosén, Johanna; Bilek, Marcela M. M.; McKenzie, David R.

2011-01-01

195

A Gas Phase in Viable Fungal Spores  

Microsoft Academic Search

WHEN the dark ascospore of Sordaria fimicola is examined dry, a round paler zone in the middle is invariably seen. If the spore is flooded with water, it is clear that this zone is a gas bubble which disappears in a few minutes. If the spore is again allowed to dry, very shortly after the last visible trace of external

C. T. Ingold

1956-01-01

196

Field driven ferromagnetic phase nucleation and propagation from the domain boundaries in antiferromagnetically coupled perpendicular anisotropy films  

SciTech Connect

We investigate the reversal process in antiferromagnetically coupled [Co/Pt]{sub X-1}/{l_brace}Co/Ru/[Co/Pt]{sub X-1}{r_brace}{sub 16} multilayer films by combining magnetometry and Magnetic soft X-ray Transmission Microscopy (MXTM). After out-of-plane demagnetization, a stable one dimensional ferromagnetic (FM) stripe domain phase (tiger-tail phase) for a thick stack sample (X=7 is obtained), while metastable sharp antiferromagnetic (AF) domain walls are observed in the remanent state for a thinner stack sample (X=6). When applying an external magnetic field the sharp domain walls of the thinner stack sample transform at a certain threshold field into the FM stripe domain wall phase. We present magnetic energy calculations that reveal the underlying energetics driving the overall reversal mechanisms.

Hauet, Thomas; Gunther, Christian M.; Hovorka, Ondrej; Berger, Andreas; Im, Mi-Young; Fischer, Peter; Hellwig, Olav

2008-12-09

197

Two-phase modeling of a gas phase polyethylene fluidized bed reactor  

Microsoft Academic Search

A two-phase model is proposed for describing the behavior of a fluidized bed reactor used for polyethylene production. In the proposed model, the bed is divided into several sequential sections where flow of the gas is considered to be plug flow through the bubbles and perfectly mixed through the emulsion phase. Polymerization reactions occur not only in the emulsion phase

Ali Kiashemshaki; Navid Mostoufi; Rahmat Sotudeh-Gharebagh

2006-01-01

198

Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation  

NASA Astrophysics Data System (ADS)

During the FROST-2 (FReezing Of duST) measurement campaign conducted at the Leipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changes in the ice nucleation properties of 300 nm Arizona Test Dust mineral particles following thermochemical processing by varying amounts and combinations of exposure to sulphuric acid vapour, ammonia gas, water vapour, and heat. The processed particles' heterogeneous ice nucleation properties were determined in both the water subsaturated and supersaturated humidity regimes at -30 °C and -25 °C using Colorado State University's continuous flow diffusion chamber. The amount of sulphuric acid coating material was estimated by an aerosol mass spectrometer and from CCN-derived hygroscopicity measurements. The condensation of sulphuric acid decreased the dust particles' ice nucleation ability in proportion to the amount of sulphuric acid added. Heating the coated particles in a thermodenuder at 250 °C - intended to evaporate the sulphuric acid coating - reduced their freezing ability even further. We attribute this behaviour to accelerated acid digestion of ice active surface sites by heat. Exposing sulphuric acid coated dust to ammonia gas produced particles with similarly poor freezing potential; however a portion of their ice nucleation ability could be restored after heating in the thermodenuder. In no case did any combination of thermochemical treatments increase the ice nucleation ability of the coated mineral dust particles compared to unprocessed dust. These first measurements of the effect of identical chemical processing of dust particles on their ice nucleation ability under both water subsaturated and mixed-phase supersaturated cloud conditions revealed that ice nucleation was more sensitive to all coating treatments in the water subsaturated regime. The results clearly indicate irreversible impairment of ice nucleation activity in both regimes after condensation of concentrated sulphuric acid. This implies that the sulphuric acid coating caused permanent chemical and/or physical modification of the ice active surface sites; the possible dissolution of the coating during droplet activation did not restore all immersion/condensation-freezing ability.

Sullivan, R. C.; Petters, M. D.; Demott, P. J.; Kreidenweis, S. M.; Wex, H.; Niedermeier, D.; Hartmann, S.; Clauss, T.; Stratmann, F.; Reitz, P.; Schneider, J.; Sierau, B.

2010-12-01

199

Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation  

NASA Astrophysics Data System (ADS)

During the FROST-2 (FReezing Of duST) measurement campaign conducted at the Leipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changes in the ice nucleation properties of 300 nm Arizona test dust mineral particles following thermochemical processing by varying amounts and combinations of exposure to sulphuric acid vapour, ammonia gas, water vapour, and heat. The processed aerosol's heterogeneous ice nucleation properties were determined in both the water subsaturated and supersaturated humidity regimes at -30 °C and -25 °C using Colorado State University's continuous flow diffusion chamber. The amount of sulphuric acid coating material was estimated by an aerosol mass spectrometer and from CCN-derived hygroscopicity measurements. The condensation of sulphuric acid decreased the dust particles' ice nucleation ability in proportion to the amount of sulphuric acid added. Heating the coated particles in a thermodenuder at 250 °C - intended to evaporate the sulphuric acid coating - reduced their freezing ability even further. We attribute this behaviour to accelerated acid digestion of ice active surface sites by heat. Exposing sulphuric acid coated dust to ammonia gas produced particles with similarly poor freezing potential; however a portion of their ice nucleation ability could be restored after heating in the thermodenuder. In no case did any combination of thermochemical treatments increase the ice nucleation ability of the coated mineral dust particles compared to unprocessed dust. These first measurements of the effect of identical chemical processing of dust particles on their ice nucleation ability in both water subsaturated and mixed-phase supersaturated cloud conditions revealed that ice nucleation was more sensitive to all coating treatments in the water subsaturated regime. The results clearly indicate irreversible impairment of ice nucleation activity in both regimes after condensation of concentrated sulphuric acid. This implies that the sulphuric acid coating caused permanent chemical and/or physical modification of the ice active surface sites; the possible dissolution of the coating during droplet activation did not restore all immersion/condensation-freezing ability.

Sullivan, R. C.; Petters, M. D.; Demott, P. J.; Kreidenweis, S. M.; Wex, H.; Niedermeier, D.; Hartmann, S.; Clauss, T.; Stratmann, F.; Reitz, P.; Schneider, J.

2010-07-01

200

Hydrodynamic and gas phase axial dispersion in an air-molten salt two-phase system (molten salt oxidation reactor)  

Microsoft Academic Search

The effects of the gas velocity (0.05–0.22m\\/s) and temperature (870–970°C) on the gas holdup and the gas phase axial dispersion coefficient have been studied in a molten salt oxidation reactor (0.076m i.d.×0.653m H., air-molten sodium carbonate salt two-phase system). The gas phase holdup and the amount of the axial gas phase dispersion coefficient were experimentally evaluated by means of the

Yong-Jun Cho; Hee-Chul Yang; Hee-Chul Eun; Jae-Hyung Yoo; Joon-Hyung Kim

2005-01-01

201

Effect of initial conditions and loading path on earthquake nucleation  

Microsoft Academic Search

Previous studies of earthquake nucleation on faults with rate- and state-dependent friction show that nucleation process initially consists of a characteristic phase of localization to a limiting dimension, followed in some cases by a late-stage nucleation-zone expansion. Processes controlling the nucleation zone dimension are of interest for understanding scaling of the minimum earthquake dimensions and scaling of premonitory processes associated

Zijun Fang; James H. Dieterich; Guanshui Xu

2010-01-01

202

Observation of two-step nucleation in methane hydrates.  

PubMed

In this work we show that homogeneous nucleation of methane hydrate can, under appropriate conditions, be a very rapid process, achieved within tens of nanoseconds. In agreement with recent experimental results on different systems, we find that the nucleation of a gas hydrate crystal appears as a two-step process. It starts with the formation of disordered solid-like structures, which will then spontaneously evolve to more recognizable crystalline forms. This previously elusive first-stage state is confirmed to be post-critical in the nucleation process, and is characterized as processing reasonable short-range structure but essentially no long-range order. Its energy, molecular diffusion and local structure reflect a solid-like character, although it does exhibit mobility over longer (tens of ns) timescales. We provide insights into the controversial issue of memory effects in methane hydrates. We show that areas locally richer in methane will nucleate much more readily, and no 'memory' of the crystal is required for fast re-crystallization. We anticipate that much richer polycrystallinity and novel methane hydrate phases could be possible. PMID:20957258

Vatamanu, Jenel; Kusalik, Peter G

2010-10-18

203

Enzymatic dehalogenation of gas phase substrates with haloalkane dehalogenase.  

PubMed

Haloalkane dehalogenase is an enzyme capable of catalyzing the conversion of short-chained (C(2)-C(8)) aliphatic halogenated hydrocarbons to a corresponding primary alcohol. Because of its broad substrate specificity for mono-, di-, and trisubstituted halogenated hydrocarbons and cofactor independence, haloalkane dehalogenases are attractive biocatalysts for gas-phase bioremediation of pollutant halogenated vapor emissions. A solid preparation of haloalkane dehalogenase from Rhodococcus rhodochrous was used to catalyze the dehalogenation reaction of 1-chlorobutane or 1,3-dichloropropane delivered in the gas phase. For optimal gas-phase dehalogenase activity, a relative humidity of 100%, a(w) = 1, was desired. With a 50% reduction in the vapor-phase hydration level, an 80% decrease in enzymatic activity was observed. The enzyme kinetics for the gas-phase substrates obeyed an Arrhenius-"like" behavior and the solid haloalkane dehalogenase preparation was more thermally stable than its water-soluble equivalent. Triethylamine was added to the gaseous reaction environment in efforts to increase the rate of reaction. A tenfold increase in the dehalogenase activity for the vapor-phase substrates was observed with the addition of triethylamine. Triethylamine altered the electrostatic environment of haloalkane dehalogenase via a basic shift in local pH, thereby minimizing the effect of the pH-reducing reaction product on enzyme activity. Both organic phase and solid-state buffers were used to confirm the activating role of the altered ionization state. PMID:10861403

Dravis, B C; LeJeune, K E; Hetro, A D; Russell, A J

2000-08-01

204

Fabrication and morphologies of large directly ordered L1{sub 0} FePt nanoparticles in gas phase  

SciTech Connect

Gas phase synthesis of large directly ordered L1{sub 0} FePt nanoparticles was studied. Simultaneous control of the chemical ordering and the size of the FePt nanoparticle was successfully achieved. It was found that the chemical ordering of the FePt nanoparticles was mainly influenced by the energy conditions (thermal environments at nucleation and growth regions), which could be adjusted by varying the process parameters including the sputtering current density, the Ar gas pressure, etc. The sizes of ordered FePt nanoparticles were more related to the Fe and Pt atoms' density at the initial stage (close to target surface). Multiply twinned structures were observed in L1{sub 0} FePt nanoparticles with 12 and 17 nm mean sizes, but were absent in L1{sub 0} FePt nanoparticles with 6 nm mean size, which caused the relatively low coercivity of large L1{sub 0} FePt nanoparticles.

Liu Xiaoqi; Wang Jianping [Center for Micromagnetics and Information Technologies (MINT) and Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (United States)

2009-04-01

205

Time of Nucleation of Phase-Slip Centers in YBa2Cu3O7 Superconducting Bridges  

Microsoft Academic Search

Narrow YBa2Cu3O7 films, excited by pulses of supercritical current, had their nanosecond electric response monitored in zero applied magnetic field. Delayed voltage steps plus constant differential resistance, characteristic of phase-slip centers (PSC), are observed at all temperatures. The duration of the initial zero voltage state is well fit by Ginzburg-Landau based theories, with a gap relaxation time controlled by phonon

F. S. Jelila; J.-P. Maneval; F.-R. Ladan; F. Chibane; A. Marie-de-Ficquelmont; L. Méchin; J.-C. Villégier; M. Aprili; J. Lesueur

1998-01-01

206

Numerical analysis of gas-particle two-phase flows  

NASA Astrophysics Data System (ADS)

A numerical analysis of axisymmetric gas-particle two phase flows is described. Under-expanded supersonic free-jet flows and supersonic flows around a truncated cylinder of gas-particle mixtures are solved numerically on the Fujitsu VP-200 supercomputer. Although the gas phase is treated as a continuum medium, the particle phase is done partly as a discrete one. The particle cloud is divided into a large number of small ring clouds. In each cloud, particles are approximated to have nearly the same velocity and temperature. The particle flowfield is obtained by following these individual ring clouds separately in the whole computational domain. In estimating the time rates of momentum and heat transfers from the particle phase to the gas one, the contributions from these ring clouds are averaged over some volume whose characteristic length is sufficiently small compared with the characteristic length of the flowfield but sufficiently large compared with that of the ring clouds. The numerical results have shown that the flow characteristics of the gas-particle mixtures are very much different from those of the dust-free gas.

Ishii, Ryuji; Umeda, Yoshikuni; Uhi, Masatoshi

1988-12-01

207

SVOC partitioning between the gas phase and settled dust indoors  

NASA Astrophysics Data System (ADS)

Semivolatile organic compounds (SVOCs) are a major class of indoor pollutants. Understanding SVOC partitioning between the gas phase and settled dust is important for characterizing the fate of these species indoors and the pathways by which humans are exposed to them. Such knowledge also helps in crafting measurement programs for epidemiological studies designed to probe potential associations between exposure to these compounds and adverse health effects. In this paper, we analyze published data from nineteen studies that cumulatively report measurements of dustborne and airborne SVOCs in more than a thousand buildings, mostly residences, in seven countries. In aggregate, measured median data are reported in these studies for 66 different SVOCs whose octanol-air partition coefficients ( Koa) span more than five orders of magnitude. We use these data to test a simple equilibrium model for estimating the partitioning of an SVOC between the gas phase and settled dust indoors. The results demonstrate, in central tendency, that a compound's octanol-air partition coefficient is a strong predictor of its abundance in settled dust relative to its gas phase concentration. Using median measured results for each SVOC in each study, dustborne mass fractions predicted using Koa and gas-phase concentrations correlate reasonably well with measured dustborne mass fractions ( R2 = 0.76). Combined with theoretical understanding of SVOC partitioning kinetics, the empirical evidence also suggests that for SVOCs with high Koa values, the mass fraction in settled dust may not have sufficient time to equilibrate with the gas phase concentration.

Weschler, Charles J.; Nazaroff, William W.

2010-09-01

208

Microfabricated Gas Phase Chemical Analysis Systems  

SciTech Connect

A portable, autonomous, hand-held chemical laboratory ({micro}ChemLab{trademark}) is being developed for trace detection (ppb) of chemical warfare (CW) agents and explosives in real-world environments containing high concentrations of interfering compounds. Microfabrication is utilized to provide miniature, low-power components that are characterized by rapid, sensitive and selective response. Sensitivity and selectivity are enhanced using two parallel analysis channels, each containing the sequential connection of a front-end sample collector/concentrator, a gas chromatographic (GC) separator, and a surface acoustic wave (SAW) detector. Component design and fabrication and system performance are described.

FRYE-MASON,GREGORY CHARLES; HELLER,EDWIN J.; HIETALA,VINCENT M.; KOTTENSTETTE,RICHARD; LEWIS,PATRICK R.; MANGINELL,RONALD P.; MATZKE,CAROLYN M.; WONG,CHUNGNIN C.

1999-09-16

209

Microfabricated Gas Phase Chemical Analysis Systems  

SciTech Connect

A portable, autonomous, hand-held chemical laboratory ({mu}ChemLab{trademark}) is being developed for trace detection (ppb) of chemical warfare (CW) agents and explosives in real-world environments containing high concentrations of interfering compounds. Microfabrication is utilized to provide miniature, low-power components that are characterized by rapid, sensitive and selective response. Sensitivity and selectivity are enhanced using two parallel analysis channels, each containing the sequential connection of a front-end sample collector/concentrator, a gas chromatographic (GC) separator, and a surface acoustic wave (SAW) detector. Component design and fabrication and system performance are described.

Casalnuovo, Stephen A.; Frye-Mason, Gregory C; Heller, Edwin J.; Hietala, Vincent M.; Kottenstette, Richard J.; Lewis, Patrick R.; Manginell, Ronald P.; Matzke, Carolyn M.; Wong, C. Channy

1999-08-02

210

Improved coarse-grained model for molecular-dynamics simulations of water nucleation  

NASA Astrophysics Data System (ADS)

We developed a new coarse-grained (CG) model for water to study nucleation of droplets from the vapor phase. The resulting potential has a more flexible functional form and a longer range cutoff compared to other CG potentials available for water. This allowed us to extend the range of applicability of coarse-grained techniques to nucleation phenomena. By improving the description of the interactions between water molecules in the gas phase, we obtained CG model that gives similar results than the all-atom (AA) TIP4P model but at a lower computational cost. In this work we present the validation of the potential and its application to the study of nucleation of water droplets from the supersaturated vapor phase via molecular-dynamics simulations. The computed nucleation rates at T = 320 K and 350 K at different supersaturations, ranging from 5 to 15, compare very well with AA TIP4P simulations and show the right dependence on the temperature compared with available experimental data. To help comparison with the experiments, we explored in detail the different ways to control the temperature and the effects on nucleation.

Zipoli, Federico; Laino, Teodoro; Stolz, Steffen; Martin, Elyette; Winkelmann, Christoph; Curioni, Alessandro

2013-09-01

211

Nucleation theorems, the statistical mechanics of molecular clusters, and a revision of classical nucleation theory  

Microsoft Academic Search

The nucleation theorems relate the temperature and supersaturation dependence of the rate of nucleation of droplets from a metastable vapor phase to properties of the critical molecular cluster, the size that is approximately equally likely to grow or decay. They are derived here using a combination of statistical mechanics and cluster population dynamics, using an arbitrary model cluster definition. The

I. J. Ford

1997-01-01

212

Nucleation theorems, the statistical mechanics of molecular clusters, and a revision of classical nucleation theory  

Microsoft Academic Search

The nucleation theorems relate the temperature and supersaturation dependence of the rate of nucleation of droplets from a metastable vapor phase to properties of the critical molecular cluster, the size that is approxi- mately equally likely to grow or decay. They are derived here using a combination of statistical mechanics and cluster population dynamics, using an arbitrary model cluster definition.

I. J. Ford

1997-01-01

213

Gas Phase Chiral Separations By Ion Mobility Spectrometry  

PubMed Central

This manuscript introduces the concept of Chiral Ion Mobility Spectrometry (CIMS) and presents examples demonstrating the gas phase separation of enantiomers of a wide range of racemates including pharmaceuticals, amino acids and carbohydrates. CIMS is similar to traditional ion mobility spectrometry (IMS), where gas phase ions, when subjected to a potential gradient are separated at atmospheric pressure due to differences in their shapes and sizes. In addition to size and shape, CIMS separates ions based on their stereospecific interaction with a chiral gas. In order to achieve chiral discrimination by CIMS, an asymmetric environment was provided by doping the drift gas with a volatile chiral reagent. In this study S-(+)-2-butanol was used as a chiral modifier to demonstrate enantiomeric separations of atenolol, serine, methionine, threonine, methyl-?-glucopyranoside, glucose, penicillamine, valinol, phenylalanine, and tryptophan from their respective racemic mixtures.

Dwivedi, Prabha; Wu, Ching; Hill, Herbert H.

2013-01-01

214

Gas-phase dissociation pathways of a tetrameric protein complex  

NASA Astrophysics Data System (ADS)

The gas-phase dissociation of the tetrameric complex transthyretin (TTR) has been investigated with tandem-mass spectrometry (tandem-MS) using a nanoflow-electrospray interface and a quadrupole time-of-flight (Q-TOF) mass spectrometer. The results show that highly charged monomeric product ions dissociate from the macromolecular complex to form trimeric products. Manipulating the pressure conditions within the mass spectrometer facilitates the formation of metastable ions. These were observed for the transitions from tetrameric to monomeric and trimeric product ions and additionally for losses of small molecules associated with the protein complex in the gas phase. These results are interpreted in the light of recent mechanisms for the electrospray process and provide insight into the composition and factors governing the stability of macromolecular ions in the gas phase.

Sobott, Frank; McCammon, Margaret G.; Robinson, Carol V.

2003-12-01

215

Homogeneous nucleation and growth in supersaturated zinc vapor investigated by molecular dynamics simulation  

NASA Astrophysics Data System (ADS)

Homogeneous nucleation and growth of zinc from supersaturated vapor are investigated by nonequilibrium molecular dynamics simulations in the temperature range from 400 to 800 K and for a supersaturation ranging from log S=2 to 11. Argon is added to the vapor phase as carrier gas to remove the latent heat from the forming zinc clusters. A new parametrization of the embedded atom method for zinc is employed for the interaction potential model. The simulation data are analyzed with respect to the nucleation rates and the critical cluster sizes by two different methods, namely, the threshold method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)] and the mean first passage time method for nucleation by Wedekind et al. [J. Chem. Phys. 126, 134103 (2007)]. The nucleation rates obtained by these methods differ approximately by one order of magnitude. Classical nucleation theory fails to describe the simulation data as well as the experimental data. The size of the critical cluster obtained by the mean first passage time method is significantly larger than that obtained from the nucleation theorem.

Römer, F.; Kraska, T.

2007-12-01

216

Gas phase chemical detection with an integrated chemical analysis system  

SciTech Connect

Microfabrication technology has been applied to the development of a miniature, multi-channel gas phase chemical laboratory that provides fast response, small size, and enhanced versatility and chemical discrimination. Each analysis channel includes a sample preconcentrator followed by a gas chromatographic separator and a chemically selective surface acoustic wave detector array to achieve high sensitivity and selectivity. The performance of the components, individually and collectively, is described.

CASALNUOVO,STEPHEN A.; FRYE-MASON,GREGORY CHARLES; KOTTENSTETTE,RICHARD; HELLER,EDWIN J.; MATZKE,CAROLYN M.; LEWIS,PATRICK R.; MANGINELL,RONALD P.; BACA,ALBERT G.; HIETALA,VINCENT M.

2000-04-12

217

Effect of phase behavior on bypassing in enriched gas floods  

SciTech Connect

Enriched gas floods incorporate a complex interaction of heterogeneity, fingering, multiphase flow, and phase behavior. Experiments and simulations indicate that the optimum solvent enrichment in high-viscosity-ratio secondary gas floods can be below minimum miscibility enrichment (MME). The compositional path and resulting mobility profile in multidimensional multiple-contact miscible (MCM) or immiscible floods are different from their 1D counterparts for high-viscosity-ratio floods in heterogeneous media.

Burger, J.E.; Bhogeswara, R.; Mohanty, K.K. (Univ. of Houston, TX (United States))

1994-05-01

218

A condensed gas–phase chemical model and its application  

Microsoft Academic Search

Using the “lumped mechanism” and “counting species” methods, we developed a condensed gas-phase chemical model based on a\\u000a simplified one. The modified quasi-steady-state approximation (QSSA) scheme and the error redistribution mass conservation\\u000a technique are adopted to solve the atmospheric chemistry kinetic equations. Results show that the condensed model can well\\u000a simulate concentration variations of gas species such as SO2, NOX,

Tijian Wang; Zhaobo Sun; Zongkai Li

1999-01-01

219

Improving gas separation properties of polymeric membranes based on glassy polymers by gas phase fluorination  

Microsoft Academic Search

The application area of existing gas separation membranes is limited by commercially available polymers for their preparation. In many cases the separation selectivity of these polymers is not sufficient for effective separation processes. One of the ways to improve the separation effectivity of existing membranes from glassy polymers is gas phase fluorination. It is very important to note that this

D. A. Syrtsova; A. P. Kharitonov; V. V. Teplyakov; G.-H. Koops

2004-01-01

220

CHEMKIN2. General Gas-Phase Chemical Kinetics  

SciTech Connect

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary, user-specified chemical reaction mechanism. A data file is generated which forms a link to the Gas-phase Subroutine Library, a collection of about 200 modular subroutines which may be called to return thermodynamic properties, chemical production rates, derivatives of thermodynamic properties, derivatives of chemical production rates, or sensitivity parameters. Both single and double precision versions of CHEMKIN are included. Also provided is a set of FORTRAN subroutines for evaluating gas-phase transport properties such as thermal conductivities, viscosities, and diffusion coefficients. These properties are an important part of any computational simulation of a chemically reacting flow. The transport properties subroutines are designed to be used in conjunction with the CHEMKIN Subroutine Library. The transport properties depend on the state of the gas and on certain molecular parameters. The parameters considered are the Lennard-Jones potential well depth and collision diameter, the dipole moment, the polarizability, and the rotational relaxation collision number.

Rupley, F.M. [Sandia National Labs., Livermore, CA (United States)

1992-01-24

221

CHEMKIN2. General Gas-Phase Chemical Kinetics  

SciTech Connect

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. CHEMKIN was designed as an easily transportable program for use on CDC, Cray, and DEC VAX systems. It consists of two major components: the Interpreter and the Gas-phase Subroutine Library. The Interpreter reads a symbolic description of an arbitrary, user-specified chemical reaction mechanism. A data file is generated which forms a link to the Gas-phase Subroutine Library, a collection of about 200 modular subroutines which may be called to return thermodynamic properties, chemical production rates, derivatives of thermodynamic properties, derivatives of chemical production rates, or sensitivity parameters. Both single and double precision versions of CHEMKIN are included. Also provided is a set of FORTRAN subroutines for evaluating gas-phase transport properties such as thermal conductivities, viscosities, and diffusion coefficients. These properties are an important part of any computational simulation of a chemically reacting flow. The transport properties subroutines are designed to be used in conjunction with the CHEMKIN Subroutine Library. The transport properties depend on the state of the gas and on certain molecular parameters. The parameters considered are the Lennard-Jones potential well depth and collision diameter, the dipole moment, the polarizability, and the rotational relaxation collision number.

Kee, R.J. [Sandia National Labs., Livermore, CA (United States); Warnatz, J. [Institut fuer Angewandte Physikalische Chemie, Universitaet Heidleberg, Heidleberg (Germany)

1980-03-01

222

Gas phase infrared spectra and corresponding DFT calculations of ?, ?-diphenylpolyenes  

NASA Astrophysics Data System (ADS)

We present gas phase Fourier Transform Infrared (FTIR) spectra of the homologue series of ?, ?-diphenylpolyenes consisting of trans- and cis-stilbene, diphenylbutadiene (DPB) and diphenylhexatriene (DPH) obtained by a fast thermal heating technique that enables vaporization without decomposition. Infrared marker bands for the cis-isomers of the polyenes have been identified by density functional calculations at the B3LYP/TZVP level of theory. The all trans isomers of DPB and DPH do not interconvert to the cis-isomers in the gas phase at 200 °C.

Biemann, Lars; Braun, Michaela; Kleinermanns, Karl

2010-01-01

223

Calculation of two-phase flow in gas turbine combustors  

SciTech Connect

A method is presented for computing steady two-phase turbulent combusting flow in a gas turbine combustor. The gas phase equations are solved in an Eulerian frame of reference. The two-phase calculations are performed by using a liquid droplet spray combustion a model and treating the motion of the evaporating fuel droplets in a Lagrangian frame of reference. The numerical algorithm employs nonorthogonal curvilinear coordinates, a multigrid iterative solution procedure, the standard k-{epsilon} turbulence model, and a combustion model comprising an assumed shape probability density function and the conserved scalar formulation. The trajectory computation of the fuel provides the source terms for all the gas phase equations. This two-phase model was applied to a real piece of combustion hardware in the form of a modern GE/SNECMA single annular CFM56 turbofan engine combustor. For the purposes of comparison, calculations were also performed by treating the fuel as a single gaseous phase. The effect on the solution of two extreme situations of the fuel as a gas and initially as a liquid was examined. The distribution of the velocity field and the conserved scalar within the combustor, as well as the distribution of the temperature field in the reaction zone and in the exhaust, were all predicted with the combustor operating both at high-power and low-power (ground idle) conditions. The calculated exit gas temperature was compared with test rig measurements. Under both low and high-power conditions, the temperature appeared to show an improved agreement with the measured data when the calculations were performed with the spray model as compared to a single-phase calculation.

Tolpadi, A.K. [General Electric Corporate Research and Development, Schenectady, NY (United States). Fluid Mechanics Program

1995-10-01

224

Ion induced nucleation in the atmosphere: Studies of ammonia, sulfuric acid, and water cluster ions  

NASA Astrophysics Data System (ADS)

Current models of homogeneous gas phase nucleation lack thermodynamic data essential to understanding particle growth mechanisms and accurately predicting nucleation rates in the atmosphere. Nucleation model predictions are highly sensitive to the thermodynamics of the initial stages of particle growth, where free energies calculated using classical nucleation theory are expected to be the least accurate. There is strong evidence that H2SO 4, H2O, and NH3 are involved in atmospheric nucleation. These species are also principal components of atmospheric ions, suggesting that they may play important roles in ion-induced nucleation mechanisms. The purpose of this work is to measure thermodynamic parameters for cluster ions composed of NH3, H2SO4, and H2O. Equilibrium constants for the reactions of H2O and NH3 with the cluster ions, H+(H2SO4) s(H2O)w, HSO4-(H 2SO4)s(H2O) w, and NH4+(NH3) n(H2SO4)s(H 2O)w, were measured using an ion flow reactor over a range of temperatures, and thermodynamic quantities (Delta H° and DeltaS°) were derived from van't Hoff analyses. Ab initio structures were calculated to elucidate trends in ligand bonding. Current experimental results, combined with our previous measurements of H2SO4 bond enthalpies within cluster ions, are used to determine the nucleating ability of these systems for a range of atmospheric conditions. In general, nucleation of the negative cluster ions is predicted to be preferred over nucleation of positive ions in the atmosphere. For temperatures ?250 K, relative humidities ?50%, and H2SO4 concentrations ?107 molecule cm-3 there is no thermodynamic barrier to nucleation of the HSO4-(H2SO4) s(H2O)w system. Ammonia typically stabilizes H2SO4 within positive cluster ions due to formation of fully ionic crystalline-like clusters of the type NH 4+(NH4+HSO4 -)c, c = 1--3. However, nucleation barriers for the NH4+(NH 3)n(H2SO4) s(H2O)w system are strongly dependent on ambient NH3 concentrations, and positive ion nucleation in the atmosphere may actually be inhibited by the presence of low levels of NH3.

Froyd, Karl David

225

Gas-phase azide functionalization of carbon.  

PubMed

Tailoring the surface and interfacial properties of inexpensive and abundant carbon materials plays an increasingly important role for innovative applications including those in electrocatalysis, energy storage, gas separations, and composite materials. Described here is the novel preparation and subsequent use of gaseous iodine azide for the azide modification of carbon surfaces. In-line generation of gaseous iodine azide from iodine monochloride vapor and solid sodium azide is safe and convenient. Immediate treatment of carbon surfaces with this gaseous stream of iodine azide provides a highly reproducible, selective, and scalable azide functionalization that minimizes waste and reduces deleterious side reactions. Among the possible uses of azide-modified surfaces, they serve as versatile substrates for the attachment of additional functionality by coupling with terminal alkynes under the mild copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. For instance, coupling ethynylferrocene to azide-modified glassy carbon surfaces achieves ferrocene coverage up to 8 × 10(13) molecules/cm(2) by voltammetric and XPS analyses. The 1,2,3-triazole linker formed during the CuAAC reaction is robust and hydrolytically stable in both aqueous 1 M HClO(4) and 1 M NaOH for at least 12 h at 100 °C. PMID:23301920

Stenehjem, Eric D; Ziatdinov, Vadim R; Stack, T Daniel P; Chidsey, Christopher E D

2013-01-09

226

Energy partitioning in elementary gas phase reactions  

SciTech Connect

During the last 2{1/2} years we: Measured the LIF excitation spectrum of H atoms dissociated by 193 nm light from cyclopentadiene and indene as well as the rate of dissociation of indene. A general relation is that the translational temperature of the H atoms and the vibrational temperature of the molecules before dissociation are equal. We measured the H atom channel in the photodissociation of ethylene at 193 nm. The LIF excitation spectrum was measured as well as the quantum yield. Isotope effects were observed and explained by RRKM theory in CH{sub 2}CD{sub 2} and trans-CHDCHD. We studied the photodissociation of hydrogen molecules with two photon excited states of the rare gas atoms, 5p(01/2) of Kr and 6p(01/2) of Xe. A new much stronger source of Lyman alpha light at 121.6 nm enabled the study of processes in which a hydride is photodissociated at 121.6 nm and within the same pulse a second photon excites H atom fluorescence. For the first time the dynamics of photodissociation of methane, water, acetylene and HCl were measured at 121.6 nm. Large isotope effects were seen. In CH{sub 3}D, CH{sub 2}D{sub 2} and CD{sub 3}H the H atoms were twice as likely to dissociate as the D atoms.

Bersohn, R.

1991-01-01

227

Gas phase radiative effects in diffusion flames  

NASA Astrophysics Data System (ADS)

Several radiation models are evaluated for a stagnation point diffusion flame of a solid fuel in terms of accuracy and computational time. Narrowband, wideband, spectral line weighted sum of gray gases (SLWSGG), and gray gas models are included in the comparison. Radiative heat flux predictions by the nongray narrowband, wideband, and SLWSGG models are found to be in good agreement with each other, whereas the gray gas models are found to be inaccurate. The narrowband model, the most complex among the models evaluated, is then applied first to a solid fuel and second to a pure gaseous diffusion flame. A polymethylmethacrylate (PMMA) diffusion flame in a stagnation point geometry is solved with the narrowband model with COsb2, Hsb2O, and MMA vapor included in participating species. A detailed account of the emission and absorption from these species as well as the radiative heat fluxes are given as a function of the stretch rate. It is found that at low stretch rate the importance of radiation is increased due to an increase in the optical thickness, and a decrease in the conductive heat flux. Results show that COsb2 is the biggest emitter and absorber in the flame, MMA vapor is the second and Hsb2O is the least important. A pure gaseous flame in an opposed jet configuration is solved with the narrowband radiation model with CO as the fuel, and Osb2 as the oxidizer. Detailed. chemical kinetics and transport are incorporated into the combustion model with the use of the CHEMKIN and TRANSPORT software packages. The governing equations are solved with a modified version of the OPPDIF code. Dry and wet CO flames as well as COsb2 dilution are studied. Comparison of the results with and without the consideration of radiation reveals that the radiation is important for the whole flammable range of dry CO flames and for the low stretch rates of wet flames. Without the consideration of radiation the temperature and the species mole fractions (especially of minor species) predictions are different in comparison to the results with the inclusion of the radiation in the model. A flammability map with added Hsb2O fraction and stretch rate as coordinates is drawn. The flammability map contains a blow off extinction boundary at high stretch rate and a quenching extinction boundary at low stretch rate. With increasing Hsb2O addition the quenching boundary shifts to lower stretch rates and the blow off boundary shifts to higher stretch rates, hence the range of flammable stretch rates increases. COsb2 dilution of the fuel (CO) jet is found to decrease the flame temperature. A flammability map with COsb2 mole fraction in the fuel jet and the stretch rate as coordinates is also drawn. With increasing COsb2 dilution the range of flammable stretch rate decreases. For the dry CO-Osb2 case, when COsb2 dilution exceeds 17%, the system is nonflammable for any stretch rate. The most flammable stretch rate is around 2 ssp{-1}.

Bedir, Hasan

228

Nucleation of an oil phase in a nonionic microemulsion-containing chlorinated oil upon systematic temperature quench.  

PubMed

A clear and stable nonionic model microemulsion consisting of pentaoxyethylene glycol dodecyl ether (C(12)E(5)), water, and 1-chlorotetradecane (CLTD) was prepared. This system was subjected to a systematic temperature quench (perturbation out of equilibrium) in steps of 1.0 degrees C from 20.4 to 15.3 degrees C in the unstable region of its phase diagram. The change in turbidity (for droplet volume fractions of 0.02 and 0.08) and hydrodynamic radius (R(h)) (for a droplet volume fraction of 0.02) of the system on its way to its new equilibrium was measured at each quench temperature. For small systematic temperature quenches just below the emulsification failure boundary (EFB) the turbidity decreases and remains constant indicating quick changes in the microstructures. Further lowering of temperature brings the system to the unstable region where the turbidity and light scattering increase sharply as function of time because of expulsion of excess oil from the microemulsion droplets. The newly formed oil-rich droplets grow in size as a function of time. These observations indicate the existence of a narrow but observable metastable region en route to the new equilibrium where both microemulsion droplets and larger oil-rich droplets coexist. The region in which microemulsion droplets are metastable is very narrow and is concentration-dependent. The presence of a metastable region is as for other similar systems attributed to the presence of a free energy barrier for the formation of the larger oil-rich droplets associated with curvature free energy of the surfactant film. The turbidity-time curves were converted to the radius-time curves using a model assuming monodisperse spherical droplets. The obtained results are in good agreement with the results for the hydrodynamic radius. The observed average radius from both type of measurements decreases in the metastable region. By performing calculation of the influence of eccentricity and size polydispersity on the observed radius, we have shown that the distribution of the microemulsion droplets becomes more homogeneous in the metastable region. PMID:20491492

Deen, G Roshan; Pedersen, Jan Skov

2010-06-17

229

On chip steady liquid-gas phase separation for flexible generation of dissolved gas concentration gradient.  

PubMed

In this study, steady liquid-gas phase separation is realized by applying a hydrophobic small microchannel array (SMA) to bridge two large microchannels, one for liquid phase and one for gas phase. In this structure, a capillary pressure difference between that in the SMA and the larger channel results in a steady liquid-gas interface. The generated liquid-gas interface allows for fast gas dissolving speed. By coupling the liquid-gas interface with a one directional fluidic field, a steady dissolved gas concentration gradient (DgCG) is generated. The DgCG distribution is easily designable for linear or exponential modes, providing improved flexibility for gas participated processes on chip. To demonstrate its applicability, a CO(2) DgCG chip is fabricated and applied for screening CaCO(3) crystal growth conditions in the DgCG chip. Crystals with transitional structures are successfully fabricated, which is consistent with the CO(2) DgCG distribution. PMID:22336913

Xu, Bi-Yi; Hu, Shan-Wen; Yan, Xiao-Na; Xia, Xing-Hua; Xu, Jing-Juan; Chen, Hong-Yuan

2012-02-15

230

Ice-Nucleating Bacteria  

NASA Astrophysics Data System (ADS)

Since the discovery of ice-nucleating bacteria in 1974 by Maki et al., a large number of studies on the biological characteristics, ice-nucleating substance, ice nucleation gene and frost damage etc. of the bacteria have been carried out. Ice-nucleating bacteria can cause the freezing of water at relatively warm temperature (-2.3°C). Tween 20 was good substrates for ice-nucleating activity of Pseudomonas fluorescens KUIN-1. Major fatty acids of Isolate (Pseudomonas fluorescens) W-11 grown at 30°C were palmitic, cis-9-hexadecenoic and cis-11-octadecenoic which amounted to 90% of the total fatty acids. Sequence analysis shows that an ice nucleation gene from Pseudomonas fluorescens is related to the gene of Pseudomonas syringae.

Obata, Hitoshi

231

Surface and gas phase temperatures of a tungsten coil atomizer  

NASA Astrophysics Data System (ADS)

The electrothermal atomization of elements is critically affected by the volatility of the analyte and its compounds, the matrix constituents, the atomizer material and the surface and gas phase temperatures. There are scarce temperature data about the tungsten coil atomizer (TCA). This paper involves measurements of surface and gas phase temperatures of a TCA (Osram 150 W). Three strategies were adopted to assess the surface temperature: electric current measurements in each applied voltage to calculate resistivity and correlation of this latter parameter with temperature; melting points of organic and inorganic compounds in the 120-800 °C range, and optical pyrometer measurements in the 1000-3000 °C range. The first two methods led to at least estimate values and more accurate results were attained with the optical pyrometer. The gas phase temperature was estimated based on the two-line absorption method using tin as thermometric element. The data showed a pronounced difference between surface and gas phase temperatures that can reach values higher than 1000 °C depending on the applied voltage and observation height. Data are discussed considering thermochemical processes in a double layer tungsten coiled filament.

Queiroz, Zilvanir F.; Oliveira, Pedro V.; Nóbrega, Joaquim A.; Silva, C?´ntia S.; Rufini, Iolanda A.; de Sousa, Samuel Simião; Krug, Francisco J.

2002-11-01

232

Apparatus for the gas phase combustion of liquid fuels  

Microsoft Academic Search

An arrangement to produce high efficiency gas phase combustion of liquid fuels is described. A liquid fuel, such as conventional heating oil, is pumped through a heat exchanger immersed in a liquid bath whose temperature is maintained at a level which is sufficiently high to cause the fuel to gassify but low enough to preclude undesirable chemical decomposition. The liquid

G. A. Roffe; H. A. Trucco

1977-01-01

233

Apparatus for the premixed gas phase combustion of liquid fuels  

Microsoft Academic Search

This invention relates to improvements in the art of liquid fuel combustion and, more particularly, concerns a method and apparatus for the controlled gasification of liquid fuels, the thorough premixing of the then gasified fuel with air and the subsequent gas-phase combustion of the mixture to produce a flame substantially free of soot, carbon monoxide, nitric oxide and unburned fuel.

G. A. Roffe; H. A. Trucco

1981-01-01

234

Gas-phase silicon micromachining with xenon difluoride  

Microsoft Academic Search

Xenon difluoride is a gas phase, room temperature, isotropic silicon etchant with extremely high selectivity to many materials commonly used in microelectromechancial systems, including photoresists, aluminum, and silicon dioxide. Using a simple vacuum system, the effects of etch aperture and loading were explored for etches between 10 and 200 micrometers . Etch rates as high as 40 micrometers \\/minute were

Floy I. Chang; Richard Yeh; Gisela Lin; Patrick B. Chu; Eric G. Hoffman; Ezekiel J. Kruglick; Kristofer S. Pister; Michael H. Hecht

1995-01-01

235

LOW COST IMAGER FOR POLLUTANT GAS LEAK DETECTION - PHASE II  

EPA Science Inventory

An inexpensive imaging Instrument to quickly locate leaks of methane and other greenhouse and VOC gases would reduce the cost and effort expended by industry to comply with EPA regulations. In Phase I, of this WBIR program, a new gas leak visualization camera was demonstrated...

236

The application of monoliths for gas phase catalytic reactions  

Microsoft Academic Search

A general introductory review of the fundamental principles of monoliths as supports for catalytic gas phase reactions is presented. Monoliths are used because of low pressure drop and high mechanical strength required for the harsh conditions encountered in environmental applications. The chemical and physical properties of monoliths and the basics for mass transfer calculations and pressure drop are presented. Existing

Ronald M. Heck; Suresh Gulati; Robert J. Farrauto

2001-01-01

237

Formation and dissociation processes of gas-phase detergent micelles.  

PubMed

Growing interest in micelles to protect membrane complexes during the transition from solution to gas phase prompts a better understanding of their properties. We have used ion mobility mass spectrometry to separate and assign detergent clusters formed from the n-trimethylammonium bromide series of detergents. We show that cluster size is independent of detergent concentration in solution, increases with charge state, but surprisingly decreases with alkyl chain length. This relationship contradicts the thermodynamics of micelle formation in solution. However, the liquid drop model, which considers both the surface energy and charge, correlates extremely well with the experimental cluster size. To explore further the properties of gas-phase micelles, we have performed collision-induced dissociation on them during tandem mass spectrometry. We observed both sequential asymmetric charge separation and neutral evaporation from the precursor ion cluster. Interestingly, however, we also found markedly different dissociation pathways for the longer alkyl chain detergents, with significantly fewer intermediate ions formed than for those with a shorter alkyl chain. These experiments provide an essential foundation for understanding the process of the gas-phase analysis of membrane protein complexes. Moreover they imply valuable mechanistic details of the protection afforded to protein complexes by detergent clusters during gas-phase activation processes. PMID:22512598

Borysik, Antoni J; Robinson, Carol V

2012-04-26

238

CHEMKIN2. General Gas-Phase Chemical Kinetics  

Microsoft Academic Search

CHEMKIN is a high-level tool for chemists to use to describe arbitrary gas-phase chemical reaction mechanisms and systems of governing equations. It remains, however, for the user to select and implement a solution method; this is not provided. CHEMKIN was designed as an easily transportable program for use on CDC, Cray, and DEC VAX systems. It consists of two major

R. J. Kee; J. Warnatz

1980-01-01

239

Ion-Molecule Reactions in Gas Phase Radiation Chemistry.  

ERIC Educational Resources Information Center

|Discusses some aspects of the radiation chemistry of gases, focusing on the ion-molecule and charge neutralization reactions which set study of the gas phase apart. Uses three examples that illustrate radiolysis, describing the radiolysis of (1) oxygen, (2) carbon dioxide, and (3) acetylene. (CS)|

Willis, Clive

1981-01-01

240

GAS PHASE CHEMISTRY OF SULFONATE ANIONS: BASICITIES AND FRAGMENTATION REACTIONS  

Microsoft Academic Search

The gas phase relative acidities of 15 sulfonic acids, RSO3H, have been determine via Cooks' kinetic method. The following acidity scale was established (starting with the weakest acid): methane sulfonic acid < ethane sulfonic acid < methoxy sulfonic acid < sulfuric acid < m-aminobenzene sulfonic acid - p-aminobenzene sulfonic acids < p-toluene sulfonic acid < benzene sulfonic acid < o-aminobenzene

Jacob D. Smith; Richard A. J. Ohair; Todd D. Williams

1996-01-01

241

New Spectroscopic Methods in Gas-Phase Chemical Kinetics  

Microsoft Academic Search

The possibilities provided by the application of advances in laser technology to the solution of problems in gas-phase chemical kinetics have been analysed. The basic principles of various laser methods for producing and recording active chemical species have been described. Examples of the use of lasers to study the reactions of radicals and excited species, in the diagnostics of complex

Oleg M. Sarkisov; S. G. Cheskis

1985-01-01

242

Evaluated Chemical Kinetic Rate Constants for Various Gas Phase Reactions  

Microsoft Academic Search

The available information, up to mid-1972, for the rate constants of a series of gas phase chemical reactions has been evaluated critically. For each reaction, relevant thermodynamic data are presented and values for the equilibrium constant expressed in mathematical form. Kinetic data are presented in tabular and graphical form together with a discussion of the pertinent details. Recommended rate constant

Keith Schofield

1973-01-01

243

Weak intermolecular interactions in gas-phase nuclear magnetic resonance.  

PubMed

Gas-phase nuclear magnetic resonance (NMR) spectra demonstrating the effect of weak intermolecular forces on the NMR shielding constants of the interacting species are reported. We analyse the interaction of the molecular hydrogen isotopomers with He, Ne, and Ar, and the interaction in the He-CO(2) dimer. The same effects are studied for all these systems in the ab initio calculations. The comparison of the experimental and computed shielding constants is shown to depend strongly on the treatment of the bulk susceptibility effects, which determine in practice the pressure dependence of the experimental values. Best agreement of the results is obtained when the bulk susceptibility correction in rare gas solvents is evaluated from the analysis of the He-rare gas interactions, and when the shielding of deuterium in D(2)-rare gas systems is considered. PMID:21895188

Garbacz, Piotr; Piszczatowski, Konrad; Jackowski, Karol; Moszynski, Robert; Jaszu?ski, Micha?

2011-08-28

244

Molecular gas phase counterparts to solid state grain mantles features  

NASA Astrophysics Data System (ADS)

We present ISO-SWS observations of the dense part surrounding the protostellar object RAFGL7009S. In this study, we concentrate on simple molecules detected in the gas and ice state (H_2O, CO_2 and CH_4) with the SWS(AOT06). Estimates are derived for their corresponding gas-to-solid ratios and lead to fractions around 0.2 for both H_2$O and CH_4 and almost an order of magnitude lower for CO_2. The gas phase temperature for these species in the gas is dominated by a rather cold component (~50 K). We discuss briefly the possible implications for the chemistry in this region.

Dartois, E.; D'Hendecourt, L.; Boulanger, F.; Puget, J. L.; Jourdain de Muizon, M.; Breitfellner, M.; Habing, H. J.

245

Preconceptual design of the gas-phase decontamination demonstration cart  

SciTech Connect

Removal of uranium deposits from the interior surfaces of gaseous diffusion equipment will be a major portion of the overall multibillion dollar effort to decontaminate and decommission the gaseous diffusion plants. Long-term low-temperature (LTLT) gas-phase decontamination is being developed at the K-25 Site as an in situ decontamination process that is expected to significantly lower the decontamination costs, reduce worker exposure to radioactive materials, and reduce safeguard concerns. This report documents the preconceptual design of the process equipment that is necessary to conduct a full-scale demonstration of the LTLT method in accordance with the process steps listed above. The process equipment and method proposed in this report are not intended to represent a full-scale production campaign design and operation, since the gas evacuation, gas charging, and off-gas handling systems that would be cost effective in a production campaign are not cost effective for a first-time demonstration. However, the design presented here is expected to be applicable to special decontamination projects beyond the demonstration, which could include the Deposit Recovery Program. The equipment will therefore be sized to a 200 ft size 1 converter (plus a substantial conservative design margin), which is the largest item of interest for gas phase decontamination in the Deposit Recovery Program. The decontamination equipment will allow recovery of the UF{sub 6}, which is generated from the reaction of ClF{sub 3} with the uranium deposits, by use of NaF traps.

Munday, E.B.

1993-12-01

246

Lattice-gas models of phase separation: interfaces, phase transitions, and multiphase flow  

SciTech Connect

Momentum-conserving lattice gases are simple, discrete, microscopic models of fluids. This review describes their hydrodynamics, with particular attention given to the derivation of macroscopic constitutive equations from microscopic dynamics. Lattice-gas models of phase separation receive special emphasis. The current understanding of phase transitions in these momentum-conserving models is reviewed; included in this discussion is a summary of the dynamical properties of interfaces. Because the phase-separation models are microscopically time irreversible, interesting questions are raised about their relationship to real fluid mixtures. Simulation of certain complex-fluid problems, such as multiphase flow through porous media and the interaction of phase transitions with hydrodynamics, is illustrated.

Rothman, D.H. (Laboratoire de Physique Statistique, Centre National de la Recherche Scientifique, Ecole Normale Superieure, 75005 Paris (France)); Zaleski, S. (Laboratoire de Modelisation en Mecanique, Centre National de la Recherche Scientifique, Universite Pierre et Marie Curie, 75005 Paris (France))

1994-10-01

247

Analysis of vibrational nonequilibrium of gas phase in gas-particle nozzle flows  

NASA Astrophysics Data System (ADS)

A numerical study of the simultaneous relaxation of phase nonequilibrium and vibrational nonequilibrium of the gas phase in gas-particle nozzle flows is presented. The one-dimensional unsteady governing equations are solved using a time-marching finite-difference technique. A computer program is developed which gives steady-state solutions starting from the assumed initial value distributions for the flow field parameters. The steady-state results are presented in the form of graphs for contour and hyperbolic nozzles. It is shown that the velocity and temperature slips between the gas and particle phases at low loading ratios increase with the increase of the particle size. The flow becomes near equilibrium for particle diameters in the range below 2 microns.

Reddy, N. M.; Reddy, K. P. J.; Baruah, C. K.

1990-08-01

248

FORMATION OF POLYCYCLIC AROMATIC HYDROCARBONS AND CARBONACEOUS SOLIDS IN GAS-PHASE CONDENSATION EXPERIMENTS  

SciTech Connect

Carbonaceous grains represent a major component of cosmic dust. In order to understand their formation pathways, they have been prepared in the laboratory by gas-phase condensation reactions such as laser pyrolysis and laser ablation. Our studies demonstrate that the temperature in the condensation zone determines the formation pathway of carbonaceous particles. At temperatures lower than 1700 K, the condensation by-products are mainly polycyclic aromatic hydrocarbons (PAHs) that are also the precursors or building blocks for the condensing soot grains. The low-temperature condensates contain PAH mixtures that are mainly composed of volatile three to five ring systems. At condensation temperatures higher than 3500 K, fullerene-like carbon grains and fullerene compounds are formed. Fullerene fragments or complete fullerenes equip the nucleating particles. Fullerenes can be identified as soluble components. Consequently, condensation products in cool and hot astrophysical environments such as cool and hot asymptotic giant branch stars or Wolf-Rayet stars should be different and should have distinct spectral properties.

Jaeger, C.; Huisken, F.; Henning, Th. [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany); Mutschke, H.; Jansa, I. Llamas [Astrophysikalisches Institut und Universitaets-Sternwarte (AIU), Friedrich-Schiller-Universitaet Jena, Schillergaesschen 2-3, D-07745 Jena (Germany)], E-mail: Cornelia.Jaeger@uni-jena.de

2009-05-01

249

Diamond nucleation using polyethene  

DOEpatents

The invention presents a simple, non-destructive and non-abrasive method of diamond nucleation using polyethene. It particularly describes the nucleation of diamond on an electrically viable substrate surface using polyethene via chemical vapor deposition (CVD) technique in a gaseous environment.

Morell, Gerardo; Makarov, Vladimir; Varshney, Deepak; Weiner, Brad

2013-07-23

250

The Vacuum Bubble Nucleation  

SciTech Connect

We study the nucleation of a vacuum bubble via the vacuum-to-vacuum tunneling transition in curved spacetime. We consider Coleman-de Luccia's semiclassical approximation at zero temperature in pure Einstein theory of gravity and the theory with nonminimal coupling. We discuss the dynamics of a nucleated vacuum bubble.

Lee, Bum-Hoon [Department of Physics and Center for Quantum Spacetime, Sogang University Seoul 121-742 (Korea, Republic of); Lee, Wonwoo [Center for Quantum Spacetime, Sogang University, Seoul 121-742 (Korea, Republic of)

2009-07-10

251

Gas Phase Chemical Detection with an Integrated Chemical Analysis System  

SciTech Connect

Microfabrication technology has been applied to the development of a miniature, multi-channel gas phase chemical laboratory that provides fast response, small size, and enhanced versatility and chemical discrimination. Each analysis channel includes a sample concentrator followed by a gas chromatographic separator and a chemically selective surface acoustic wave detector array to achieve high sensitivity and selectivity. The performance of the components, individually and collectively, is described. The design and performance of novel micromachined acoustic wave devices, with the potential for improved chemical sensitivity, are also described.

Baca, Albert G.; Casalnuovo, Stephen A.; Frye-Mason, Gregory C.; Heller, Edwin J.; Hietala, Susan L.; Hietala, Vincent M.; Kottenstette, Richard J.; Lewis, Patrick R.; Manginell, Ronald P.; Matzke, Carloyn M.; Reno, John L.; Sasaki, Darryl Y.; Schubert, W. Kent

1999-07-08

252

ELECTRON MICROSCOPY ANALYSIS OF THE INTERMEDIATE PHASES FORMED DURING THE NUCLEATION OF YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} FILM  

SciTech Connect

Recently, considerable efforts have been made in growing bi-axially aligned thick YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) films on a flexible, textured metallic substrate for electrical power applications. The BaF{sub 2} post-deposition annealing process is one of the most promising methods. To understand the nucleation and growth mechanism of YBCO in this process, extended transmission electron microscopy analysis has been made. Here, we report on the evolution of Ba-Y oxy-fluoride which is pertinent to the epitaxial YBCO nucleation process on SrTiO{sub 3} (STO) substrate.

WU,L.; ZHU,Y.; SLOVYOV,V.F.; WIESMANN,H.J.; SUENAGA,M.

2001-08-05

253

Collaborative Advanced Gas Turbine Program: Phase 1. Final report  

SciTech Connect

The Collaborative Advanced Gas Turbine (CAGT) Program is an advanced gas turbine research and development program whose goal is to accelerate the commercial availability, to within the turn of the century, of high efficiency aeroderivative gas turbines for electric power generating applications. In the first project phase, research was conducted to prove or disprove the research hypothesis that advanced aeroderivative gas turbine systems can provide a promising technology alternative, offering high efficiency and good environmental performance characteristics in modular sizes, for utility applications. This $5 million, Phase 1 research effort reflects the collaborative efforts of a broad and international coalition of industries and organizations, both public and private, that have pooled their resources to assist in this research. Included in this coalition are: electric and gas utilities, the Electric Power Research Institute, the Gas Research Institute and the principal aircraft engine manufacturers. Additionally, the US Department of Energy (DOE) and the California Energy Commission have interacted with the CAGT on both technical and executive levels as observers and sources of funding. The three aircraft engine manufacturer-led research teams participating in this research include: Rolls-Royce, Inc., and Bechtel; the Turbo Power and Marine Division of United Technologies and Fluor Daniel; and General Electric Power Generation, Stewart and Stevenson, and Bechtel. Each team has investigated advanced electric power generating systems based on their high-thrust (60,000 to 100,000 pounds) aircraft engines. The ultimate goal of the CAGT program is that the community of stakeholders in the growing market for natural-gas-fueled, electric power generation can collectively provide the right combination of market-pull and technology-push to substantially accelerate the commercialization of advanced, high efficiency aeroderivative technologies.

Hollenbacher, R.; Kesser, K.; Beishon, D.

1994-12-01

254

Carbon Dioxide Separation from Flue Gas by Phase Enhanced Absorption  

SciTech Connect

A new process, phase enhanced absorption, was invented. The method is carried out in an absorber, where a liquid carrier (aqueous solution), an organic mixture (or organic compound), and a gas mixture containing a gas to be absorbed are introduced from an inlet. Since the organic mixture is immiscible or at least partially immiscible with the liquid carrier, the organic mixture forms a layer or small parcels between the liquid carrier and the gas mixture. The organic mixture in the absorber improves mass transfer efficiency of the system and increases the absorption rate of the gas. The organic mixture serves as a transportation media. The gas is finally accumulated in the liquid carrier as in a conventional gas-liquid absorption system. The presence of the organic layer does not hinder the regeneration of the liquid carrier or recovery of the gas because the organic layer is removed by a settler after the absorption process is completed. In another aspect, the system exhibited increased gas-liquid separation efficiency, thereby reducing the costs of operation and maintenance. Our study focused on the search of the organic layer or transportation layer to enhance the absorption rate of carbon dioxide. The following systems were studied, (1) CO{sub 2}-water system and CO{sub 2}-water-organic layer system; (2) CO{sub 2}-Potassium Carbonate aqueous solution system and CO{sub 2}-Potassium Carbonate aqueous solution-organic layer system. CO{sub 2}-water and CO{sub 2}-Potassium Carbonate systems are the traditional gas-liquid absorption processes. The CO{sub 2}-water-organic layer and CO{sub 2}-Potassium Carbonate-organic layer systems are the novel absorption processes, phase enhanced absorption. As we mentioned early, organic layer (transportation layer phase) is used for the increase of absorption rate. Our study showed that the absorption rate can be increased by adding the organic layer. However, the enhanced factor is highly depended on the liquid mass transfer coefficient.

Liang Hu

2006-06-30

255

Carbon Dioxide Separation from Flue Gas By Phase Enhanced Absorption  

SciTech Connect

A new process, phase enhanced absorption, was invented. The method is carried out in an absorber, where a liquid carrier (aqueous solution), an organic mixture (or organic compound), and a gas mixture containing a gas to be absorbed are introduced from an inlet. Since the organic mixture is immiscible or at least partially immiscible with the liquid carrier, the organic mixture forms a layer or small parcels between the liquid carrier and the gas mixture. The organic mixture in the absorber improves mass transfer efficiency of the system and increases the absorption rate of the gas. The organic mixture serves as a transportation media. The gas is finally accumulated in the liquid carrier as in a conventional gas-liquid absorption system. The presence of the organic layer does not hinder the regeneration of the liquid carrier or recovery of the gas because the organic layer is removed by a settler after the absorption process is completed. In another aspect, the system exhibited increased gas-liquid separation efficiency, thereby reducing the costs of operation and maintenance. The study focused on the search of the organic layer or transportation layer to enhance the absorption rate of carbon dioxide. The following systems were studied, (1) CO{sub 2}-water system and CO{sub 2}-water-organic layer system; (2) CO{sub 2}-Potassium Carbonate aqueous solution system and CO{sub 2}-Potassium Carbonate aqueous solution-organic layer system. CO{sub 2}-water and CO{sub 2}-Potassium Carbonate systems are the traditional gas-liquid absorption processes. The CO{sub 2}-water-organic layer and CO{sub 2}-Potassium Carbonate-organic layer systems are the novel absorption processes, phase enhanced absorption. As mentioned earlier, organic layer (transportation layer phase) is used for the increase of absorption rate. The study showed that the absorption rate can be increased by adding the organic layer. However, the enhanced factor is highly dependent on the liquid mass transfer coefficient.

Liang Hu; Victor Roberts, Jr; Monica J. Wood

2005-10-01

256

Nuclear liquid-gas phase transition with finite range force  

NASA Astrophysics Data System (ADS)

The possibilities of the occurrence of liquid-gas phase transition in nuclear matter and finite nuclei are investigated in the framework of temperature-dependent Hartree-Fock theory employing the finite-range Brink-Boeker effective interaction. The equation of state at subnuclear density is obtained. It is found that for infinite nuclear matter critical temperature for the phase transition is substantially reduced because of the finite-range nature of the force as compared to those obtained using zero-range forces. When finite-size effect and Coulomb force are taken into account, critical temperature is found to be around 8 MeV.

Satpathy, L.; Mishra, M.; Nayak, R.

1989-01-01

257

Contrail formation: Homogeneous nucleation of H2SO4\\/H2O droplets  

Microsoft Academic Search

Homogeneous nucleation of sub-nanometer H2SO4\\/H2O germs, their growth and freezing probability in the cooling wake of a subsonic jet aircraft at tropopause altitude are investigated. Heteromolecular condensation, water uptake, and coagulation cause a small subset of the germs to grow into nm-sized solution droplets which overcome the Kelvin barrier. These droplets efficiently take up water vapor from the gas phase,

B. Kaercher; Th. Peter; R. Ottmann

1995-01-01

258

Superfluidity and phase transitions in a resonant Bose gas  

SciTech Connect

The atomic Bose gas is studied across a Feshbach resonance, mapping out its phase diagram, and computing its thermodynamics and excitation spectra. It is shown that such a degenerate gas admits two distinct atomic and molecular superfluid phases, with the latter distinguished by the absence of atomic off-diagonal long-range order, gapped atomic excitations, and deconfined atomic {pi}-vortices. The properties of the molecular superfluid are explored, and it is shown that across a Feshbach resonance it undergoes a quantum Ising transition to the atomic superfluid, where both atoms and molecules are condensed. In addition to its distinct thermodynamic signatures and deconfined half-vortices, in a trap a molecular superfluid should be identifiable by the absence of an atomic condensate peak and the presence of a molecular one.

Radzihovsky, Leo [Department of Physics, University of Colorado, Boulder, CO 80309 (United States); Weichman, Peter B. [BAE Systems, Advanced Information Technologies, 6 New England Executive Park, Burlington, MA 01803 (United States); Park, Jae I. [Department of Physics, University of Colorado, Boulder, CO 80309 (United States); National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305-3328 (United States)], E-mail: jae@nist.gov

2008-10-15

259

Spectroscopic studies of cold, gas-phase biomolecular ions  

NASA Astrophysics Data System (ADS)

While the marriage of mass spectrometry and laser spectroscopy is not new, developments over the last few years in this relationship have opened up new horizons for the spectroscopic study of biological molecules. The combination of electrospray ionisation for producing large biological molecules in the gas phase together with cooled ion traps and multiple-resonance laser schemes are allowing spectroscopic investigation of individual conformations of peptides with more than a dozen amino acids. Highly resolved infrared spectra of single conformations of such species provide important benchmarks for testing the accuracy of theoretical calculations. This review presents a number of techniques employed in our laboratory and in others for measuring the spectroscopy of cold, gas-phase protonated peptides. We show examples that demonstrate the power of these techniques and evaluate their extension to still larger biological molecules.

Rizzo, Thomas R.; Stearns, Jaime A.; Boyarkin, Oleg V.

260

Gas phase acidities of N-substituted amine-boranes.  

PubMed

Complexation energies and acidities of 19 primary, secondary and tertiary amine-boranes were investigated using MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) methods. Gas phase acidities for free amines were also calculated. Acidity values for studied complexes range from 327.3 to 349.1 kcal mol(-1) and the most acidic are the ones with direct connection between deprotonation center and a ?-system. Results obtained by both computational methods are in good agreement with each other and with known experimental data. Addition of BH3 increases the acidity of amines by 30 to 50 kcal mol(-1). This enhancement effect was compared to the respective effect witnessed in phosphine-boranes and traced back to changes of charge delocalization on nitrogen. A question about the structural stability of several deprotonated amine-borane anions in the gas phase was also raised. PMID:24085539

Adamson, Aiko; Guillemin, Jean-Claude; Burk, Peeter

2013-10-02

261

Gas-phase Fragmentation of Deprotonated p-Hydroxyphenacyl Derivatives  

PubMed Central

Electrospray ionization of methanolic solutions of p-hydroxyphenacyl derivatives HO-C6H4-C(O)-CH2-X (X = leaving group) provides abundant signals for the deprotonated species which are assigned to the corresponding phenolate anions ?O-C6H4-C(O)-CH2-X. Upon collisional activation in the gas phase, these anions inter alia undergo loss of a neutral “C8H6O2” species concomitant with formation of the corresponding anions X?. The energies required for the loss of neutral roughly correlate with the gas phase acidities of the conjugate acids (HX). Extensive theoretical studies performed for X = CF3COO in order to reveal the energetically most favorable pathway for the formation of neutral “C8H6O2” suggest three different routes of similar energy demands, involving a spirocyclopropanone, epoxide formation, and a diradical, respectively.

Remes, Marek; Roithova, Jana; Schroder, Detlef; Cope, Elizabeth D.; Perera, Chamani; Senadheera, Sanjeewa N.; Stensrud, Kenneth; Ma, Chi-cheng; Givens, Richard S.

2011-01-01

262

Model of boron diffusion from gas phase in silicon carbide  

SciTech Connect

Boron diffusion from the gas phase in silicon carbide is described on the basis of a two-component model. 'Shallow' boron, i.e., boron at silicon sites, is a slow component with a high surface concentration. Its diffusivity is proportional to the concentration of positively charged intrinsic point defects, which are presumably interstitial silicon atoms. 'Deep' boron, i.e., impurity-defect pairs of boron-carbon vacancy, is a fast component with lower surface concentration. The ratio between the surface concentrations of the components depends on the pressure of silicon or carbon vapors in the gas phase. The diffusion and interaction of components are described by the set of diffusion-reaction equations. The diffusion retardation observed on the concentration-profile tail is related to the capture of impurity-defect pairs and excess vacancies by traps of background impurities and defects.

Aleksandrov, O. V., E-mail: Aleksandr_ov@mail.ru [St. Petersburg State Electrotechnical University, 'LETI' (Russian Federation); Mokhov, E. N., E-mail: Mokhov@mail.ioffe.ru [Russian Academy of Sciences, Ioffe Physical Technical Institute (Russian Federation)

2011-06-15

263

Birch reduction of chlorobenzene in gas-phase hydrated electrons  

NASA Astrophysics Data System (ADS)

Reactions of gas-phase hydrated electrons (H2O)n-, n = 30-65, with chlorobenzene are investigated by Fourier transform ion cyclotron resonance mass spectrometry. The clusters lose water molecules via collision and black-body radiation induced dissociation. In addition, chemical reactions with chlorobenzene lead to formation of Cl-(H2O)n and OH-(H2O)n. While the former product is well-known from pulsed radiolysis studies in the bulk, the latter is observed for the first time in gas-phase clusters. It is indicative of a Birch reduction of the aromatic ring, leading to formation of a chloro-cyclohexadienyl radical C6H6Cl.

Cao, Yali; Höckendorf, Robert F.; Beyer, Martin K.

2008-11-01

264

Gas–liquid two-phase flow in microchannels Part I: two-phase flow patterns  

Microsoft Academic Search

Capillary gas–liquid two-phase flow occurs in increasingly more modern industrial applications. The existing relevant data are limited and are inconsistent with respect to the reported flow patterns and their transition boundaries. A systematic experimental investigation of two-phase flow patterns in microchannels was the objective of this study.Using air and water, experiments were conducted in circular microchannels with 1.1 and 1.45mm

K. A. Triplett; S. M. Ghiaasiaan; S. I. Abdel-Khalik; D. L. Sadowski

1999-01-01

265

Morphology, crystallography and kinetics of sympathetic nucleation  

SciTech Connect

The phenomenon of sympathetic nucleation was investigated, employing the Widmanstatten ..cap alpha.. precipitation in Ti-6.6 at%Cr and Ti-8.6 at%Mn alloys as a typical example. Two types of morphological arrangement, resulting from edge-to-edge and edge-to-face sympathetic nucleation are identified. The question of whether the latter arrangement is formed by branching in the style of solidification dendrities is considered. By expressing the orientation relationship of precipitates in the form of a matrix, a search was made for the relationships which permit branching with equivalent pairs of conjugate habit planes. From the results of such analyses, carried out on commonly observed orientation relationships among f.c.c., b.c.c. and h.c.p. phases, it is concluded that crystallographic constraints essentially prohibit branching in all of these systems. A comparative theoretical study of the relative nucleation kinetics of homogeneous, grain boundary and sympathetic nucleation.

Menon, E.S.K.; Aaronson, H.I.

1987-03-01

266

Dynamics of Nucleation in the Ising model  

NASA Astrophysics Data System (ADS)

While several theories have been developed to describe the kinetics of first order phase transitions, the range of applicability of each theory is not fully understood due to uncertainties in experiments and numerical difficulties in rare event simulations. In this study, we compute the decay rate of meta-stable states of the Ising model to test the validity of several existing nucleation theories. We employ advanced sampling methods to compute the nucleation rate, which spans a range over ten orders of magnitude, as a function of temperature and external field. Investigation of the critical nuclei and the pre-exponential factor reveals that nucleation in the 2d Ising model is well described by the field-theoretic model of Langer (1969). However, discrepancies between theory and numerical results are observed in the 3d Ising model. This discrepancy points to the importance of the shape of the critical nuclei to the nucleation kinetics.

Ryu, Seunghwa; Cai, Wei

2009-03-01

267

Molecular motors driven by asymmetric nucleation  

NASA Astrophysics Data System (ADS)

We study a one dimensional model of asymmetric nucleation where the phase boundaries are coupled to a load particle. Sites on the one-dimensional lattice are either empty or filled. Empty sites get filled faster if the is a filled site immediately preceding it. This model has applicability to nucleation problems where the substrate is directional. Examples include nucleation of proteins on filamentary substrates such as nucleic acids and microtubules. The hydrolysis of ATP or GTP in microfilaments such as RecA has been proposed as a mechanism of moving Halliday junctions, and can also be described qualitatively by our model. Using Monte Carlo simulations, we find mean velocities and of a load particle as function of the nucleation rates and the asymmetry parameter. Our results are compared with simple mean field approximations.

Lakhanpal, Amit; Chou, Tom

2007-03-01

268

Approaching complete peroxisome characterization by gas-phase fractionation  

Microsoft Academic Search

We examined the utility of gas-phase fractionation (GPF) in the m\\/z dimension to increase proteome coverage and reproducibility of peptide ion selection by direct microliquid chromatography\\/electrospray ionization-tandem mass spectrometry (LC\\/ESI-MS\\/MS) analysis of the peptides produced by proteolytic digestion of un- fractionated proteins from a yeast whole-cell lysate and in a peroxisomal membrane protein fraction derived from isolated yeast peroxisomes. We

Eugene C. Yi; Marcello Marelli; Hookeun Lee; Samuel O. Purvine; Ruedi Aebersold; John D. Aitchison; David R. Goodlett

2002-01-01

269

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows  

NASA Astrophysics Data System (ADS)

Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a ``steady flow rig'' model of an internal combustion engine.

Stier, B.; Koochesfahani, M. M.

270

Modeling Gas-Phase Chemistry in Cometary Atmospheres  

NASA Astrophysics Data System (ADS)

Gas-phase chemistry is central to understand the physics and chemistry of comets. Photochemistry is a major source of ions and electrons that further initiate key gas-phase reactions, leading to the plethora of molecules and atoms seen in cometary atmospheres. The relevant physico-chemical processes are identified within a modeling framework to understand observations and in situ measurements of comets (e.g., Halley, Borrelly, Hyakutake, Hale-Bopp, Tempel 1, Wild 2) and to provide valuable insights into the intrinsic properties of their nuclei. Details of these processes are presented, from the collision-dominated inner coma to the solar wind interaction region. This extensive modeling effort to investigate these important cometary processes is highly relevant to ground-based observations of comets and past, on going, and future spacecraft missions to these primitive objects.Gas-phase chemistry is central to understand the physics and chemistry of comets. Photochemistry is a major source of ions and electrons that further initiate key gas-phase reactions, leading to the plethora of molecules and atoms seen in cometary atmospheres. The relevant physico-chemical processes are identified within a modeling framework to understand observations and in situ measurements of comets (e.g., Halley, Borrelly, Hyakutake, Hale-Bopp, Tempel 1, Wild 2) and to provide valuable insights into the intrinsic properties of their nuclei. Details of these processes are presented, from the collision-dominated inner coma to the solar wind interaction region. This extensive modeling effort to investigate these important cometary processes is highly relevant to ground-based observations of comets and past, on going, and future spacecraft missions to these primitive objects.

Boice, D. C.

271

Gas-phase entropy generation during transient methanol droplet combustion  

Microsoft Academic Search

A numerical model was used to investigate gas-phase entropy generation during transient methanol droplet combustion in a low-pressure, zero-gravity, air environment.A comprehensive formulation for the entropy generation in a multi-component reacting flow is derived. Stationary methanol droplet combustion in a low ambient temperature (300 K) and a nearly quiescent atmosphere was studied and the effect of surface tension on entropy generation

Daniel N. Pope; Vasudevan Raghavan; George Gogos

2010-01-01

272

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows  

Microsoft Academic Search

Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing\\u000a schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more\\u000a than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity\\u000a fields

B. Stier; M. M. Koochesfahani

1999-01-01

273

Gas-phase degradation of organic compounds in the troposphere  

Microsoft Academic Search

The present status of knowledge of the gas-phase reactions of selected classes of volatile non-methane organic compounds (NMOCs) (alkanes, alkenes, aromatic hydrocarbons, oxygen-containing NMOCs and nitrogen-containing NMOCs) and their degradation products in the troposphere is briefly discussed. In the troposphere, NMOCs can undergo photolysis, reaction with the hydroxyl (OH) radical during daylight hours, reaction with the nitrate (NO3) radical (primarily

Roger Atkinson

1998-01-01

274

Gas-phase radiation-chemical oxidation of naphthalene  

Microsoft Academic Search

A kinetic model for the gas-phase radiation-chemical oxidation of naphthalene was developed on the basis of published data.\\u000a The principal routes of naphthalene conversion were studied with the use of the model. It was shown that the main product\\u000a of naphthalene radiolysis under the electron-beam dry scrubbing (EBDS) conditions of the removal of NO and SO2 from industrial gases is

G. V. Nichipor; G. Ya. Gerasimov

2008-01-01

275

Gas Phase Xenon131 Quadrupolar Splitting at High Magnetic Fields  

Microsoft Academic Search

At very high magnetic fields strengths (14 Tesla and higher) the xenon-131 gas phase NMR spectrum shows a well resolved quadrupolar splitting. Quadrupolar coupling between a non-spherical (S>1\\/2) nuclei and an electric field gradient will occur when the electrical isotropy of the surrounding electron cloud is disturbed. The experimental results suggest that the origin of the observed splitting in the

Thomas Meersmann

1998-01-01

276

Gas phase fractionation method using porous ceramic membrane  

DOEpatents

Flaw-free porous ceramic membranes fabricated from metal sols and coated onto a porous support are advantageously used in gas phase fractionation methods. Mean pore diameters of less than 40 .ANG., preferably 5-20 .ANG. and most preferably about 15 .ANG., are permeable at lower pressures than existing membranes. Condensation of gases in small pores and non-Knudsen membrane transport mechanisms are employed to facilitate and increase membrane permeability and permselectivity.

Peterson, Reid A. (Madison, WI); Hill, Jr., Charles G. (Madison, WI); Anderson, Marc A. (Madison, WI)

1996-01-01

277

Gas-Phase Fragmentation Analysis of Nitro-Fatty Acids  

Microsoft Academic Search

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for

Gustavo Bonacci; Eliana K. Asciutto; Steven R. Woodcock; Sonia R. Salvatore; Bruce A. Freeman; Francisco J. Schopfer

2011-01-01

278

Gas-Phase Fragmentation Analysis of Nitro-Fatty Acids  

Microsoft Academic Search

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and\\u000a nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional\\u000a understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene\\u000a groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for

Gustavo Bonacci; Eliana K. Asciutto; Steven R. Woodcock; Sonia R. Salvatore; Bruce A. Freeman; Francisco J. Schopfer

279

Evaporation rate of nucleating clusters.  

PubMed

The Becker-Döring kinetic scheme is the most frequently used approach to vapor liquid nucleation. In the present study it has been extended so that master equations for all cluster configurations are included into consideration. In the Becker-Döring kinetic scheme the nucleation rate is calculated through comparison of the balanced steady state and unbalanced steady state solutions of the set of kinetic equations. It is usually assumed that the balanced steady state produces equilibrium cluster distribution, and the evaporation rates are identical in the balanced and unbalanced steady state cases. In the present study we have shown that the evaporation rates are not identical in the equilibrium and unbalanced steady state cases. The evaporation rate depends on the number of clusters at the limit of the cluster definition. We have shown that the ratio of the number of n-clusters at the limit of the cluster definition to the total number of n-clusters is different in equilibrium and unbalanced steady state cases. This causes difference in evaporation rates for these cases and results in a correction factor to the nucleation rate. According to rough estimation it is 10(-1) by the order of magnitude and can be lower if carrier gas effectively equilibrates the clusters. The developed approach allows one to refine the correction factor with Monte Carlo and molecular dynamic simulations. PMID:22112089

Zapadinsky, Evgeni

2011-11-21

280

Critical droplets in transient and heterogeneous nucleation  

NASA Astrophysics Data System (ADS)

This thesis deals with analytical and computational aspects of the kinetics of phase transitions, the means by which a system initiates a first-order phase transition from a metastable phase. Typically, nucleation is studied in the classical regime, where the driving force to nucleation is small. In this case, thermal fluctuations create a bubble with a stable phase interior (the droplet bulk) and a thin interface. The stable phase interior has a lower free energy than the parent metastable phase, but the interface introduces an additional free energy cost. When the free energy cost of the interface is negligible compared to the free energy saved in the bulk interior, the bubble grows, initiating a decay to the stable phase as its boundary grows to encompass the entire system. We review theoretical aspects of nucleation theory and stochastic processes, with a particular focus on Langer's theory of the critical droplet. We propose and investigate an extension to Langer's theory that extends the definition of the critical droplet to regimes where the system cannot be described by a (metastable) equilibrium theory. We refer to this as the transient regime. We investigate the consequences of our proposal for systems that are described by both non-conserved and conserved order parameters. To better understand nucleation in the laboratory where perfect homogeneity cannot be achieved, we examine critical-droplet profiles and nucleation rates in systems with heterogeneities that both enhance and suppress droplet formation. We also examine aspects of heterogeneities in social systems. Our investigation concludes that in both microscopic nucleation processes and macroscopic social systems, heterogeneities play a dominant role in the observed statistics.

Schweiger, Aaron Olmsted

281

GAS PHASE SELECTIVE PHOTOXIDATION OF ALCOHOLS USING LIGHT-ACTIVATED TITANIUM DIOXIDE AND MOLECULAR OXYGEN  

EPA Science Inventory

Gas Phase Selective Oxidation of Alcohols Using Light-Activated Titanium Dioxide and Molecular Oxygen Gas phase selective oxidations of various primary and secondary alcohols are studied in an indigenously built stainless steel up-flow photochemical reactor using ultravi...

282

Carbon Dioxide Separation from Flue Gas by Phase Enhanced Absorption  

SciTech Connect

A new process, phase enhanced absorption, was invented. The method is carried out in an absorber, where a liquid carrier (aqueous solution), an organic mixture (or organic compound), and a gas mixture containing a gas to be absorbed are introduced from an inlet. Since the organic mixture is immiscible or at least partially immiscible with the liquid carrier, the organic mixture forms a layer or small parcels between the liquid carrier and the gas mixture. The organic mixture in the absorber improves mass transfer efficiency of the system and increases the absorption rate of the gas. The organic mixture serves as a transportation media. The gas is finally accumulated in the liquid carrier as in a conventional gas-liquid absorption system. The presence of the organic layer does not hinder the regeneration of the liquid carrier or recovery of the gas because the organic layer is removed by a settler after the absorption process is completed. In another aspect, the system exhibited increased gas-liquid separation efficiency, thereby reducing the costs of operation and maintenance. Our study focused on the search of the organic layer or transportation layer to enhance the absorption rate of carbon dioxide. The following systems were studied, (1) CO{sub 2}-water system and CO{sub 2}-water-organic layer system; (2) CO{sub 2}-Potassium Carbonate aqueous solution system and CO{sub 2}-Potassium Carbonate aqueous solution-organic layer system. CO{sub 2}-water and CO{sub 2}-Potassium Carbonate systems are the traditional gas-liquid absorption processes. The CO{sub 2}-water-organic layer and CO{sub 2}-Potassium Carbonate-organic layer systems are the novel absorption processes, phase enhanced absorption. As we mentioned early, organic layer is used for the increase of absorption rate, and plays the role of transportation of CO{sub 2}. Our study showed that the absorption rate can be increased by adding the organic layer. However, the enhanced factor is highly depended on the liquid mass transfer coefficients for the CO{sub 2}-water-organic layer system. For the CO{sub 2}-Potassium Carbonate aqueous solution-organic layer system, the enhanced factor is not only dependent on the liquid mass transfer coefficients, but also the chemical reaction rates.

Tim Fout

2007-06-30

283

Dynamical evolution of heterogeneous nucleation on surfaces with ideal cavities  

NASA Astrophysics Data System (ADS)

A theoretical investigation is conducted to explore the evolution behavior of heterogeneous nucleation on a solid surface with ideal conical cavities. According to the free energy characteristics of transition or a liquid vapor phase change in an ideal conical cavity, the heterogeneous nucleation process can be divided into or described as three different types, nucleation within a cavity, nucleation outside a cavity and twice-nucleation. The nucleation within or outside a cavity only has one free energy barrier, and its nucleation rate can be derived from the classical theory. The twice-nucleation is concluded to occur in some special cavities, and its kinetic characteristic is different from the classical theory. The twice-nucleation rate is mainly determined by two free energy barriers as the free energy of a cluster at metastable state is positive. As the free energy of a cluster at metastable state is negative, the nucleation rate of twice-nucleation is determined not only by two energy barriers, but also dependent of the metastable state.

Lu, J. F.; Peng, X. F.

2007-05-01

284

Off-gas recycle for long-term low temperature gas phase uranium decontamination.  

National Technical Information Service (NTIS)

In situ long-term low-temperature (LTLT) gas phase decontamination is being developed and demonstrated at the K-25 site as a technology that has the potential to substantially lower these costs while reducing criticality and safeguards concerns and worker...

R. D. Bundy D. H. Bunch E. B. Munday D. W. Simmons

1994-01-01

285

Correlation of droplet behavior with gas-phase structures in a gas turbine combustor  

NASA Astrophysics Data System (ADS)

The droplet-air interaction is studied downstream of the fuel injector-air swirler ('swirl cup') assembly of a CFM56 gas turbine combustor via phase Doppler interferometry and flow visualization. Droplet behavior is represented semi-quantitatively via a new approach using droplet velocity probability density functions and droplet size-velocity correlations. This information adds insight to the local behavior of droplets which has been disregarded because of a lack of appropriate diagnostics and the time required for post-data processing. In addition to the time-averaged droplet mean and rms velocities, droplet behavior including droplet spatial evolution and its correlation with the structures of the aerodynamic flow field, are identified. This study reveals that (1) droplet dispersion is dominated by the gas-phase flow field characteristics, (2) the droplet behavior and its evolution can be correlated with the large structures of the gas-phase flow field, and (3) intense interaction exists between the gas and dispersed phases.

Wang, H. Y.; McDonnell, V. G.; Samuelsen, G. S.

1993-06-01

286

Nucleation of Nanoparticle Superclusters from Solution  

NASA Astrophysics Data System (ADS)

Colloids of surface ligated nanoparticles (NP) often act as solutions with the NP displaying reversible temperature and solvent dependent solubility. In many cases when the nanoparticles are highly uniform, the precipitating solid is a two- or three-dimensional superlattice of the nanoparticles. Thus there is strong analogy to the phase behavior of molecular solutions, and it is reasonable to ask what controls the phase behavior of nanoparticle solutions and what is the nature of nucleation and growth of the insoluble phase? We have recently developed [1] a phenomenological model for the effective interaction potential between two ligated gold nanoparticles. In the current work, we carry out Brownian Dynamics simulations using this NP-NP interaction potential. We will report results from our simulations for both dynamics and shape of pre- nucleating and post-nucleating superclusters. [4pt] [1] S.J. Khan, F. Pierce, C.M. Sorensen and A. Chakrabarti, Langmuir, 25, 13861 (2009).

Khan, Siddique J.; Sorensen, C. M.; Chakrabarti, A.

2011-03-01

287

Nucleation in premicellar aggregation.  

PubMed

The role of a nucleating agent (cholesterol) in the premicellar aggregation of sodium lauryl sulfate (SLS), cetyltrimethylammonium bromide, Triton X-405 and Brij 35 has been investigated. The I(1)/I(3) ratios of pyrene in the solutions indicate that SLS is strongly nucleated by cholesterol, but the other detergents appear to be so to a lesser extent. However, quenching studies show that premicellar Brij 35 also forms nucleated aggregates that protect the probe from the quencher. Energy transfer data with Triton X-405 suggest that such aggregates have a more open structure than self-assembling ones and are more accessible to water. PMID:18965177

Loran, C P; Von Wandruszka, R

1991-05-01

288

Misfit dislocation nucleation in heteroepitaxy  

NASA Astrophysics Data System (ADS)

We have studied atomic mechanisms of misfit dislocation nucleation in heteroepitaxy with semiempirical potentials. Many-body mechanisms of stress relaxation are systematically investigated with Lennard-Jones potential in 2D and 3D cases. Energy barriers for dislocation nucleation are estimated using modern methods for saddle point search (Nudged Elastic Band [1], Eigenvector Following [2] and others). Moreover, new simple and effective method for transition paths searching is proposed. Based on the data the critical thickness of film is estimated as a function of film-substrate lattice misfit. Moreover, to make the study more realistic we used EAM [3] potentials in simulations of Pd/Cu and Cu/Pd systems. We show that the dislocations nucleate more easily in compressive than tensile strained films, and in fcc(111) orientation rather than in fcc(100). These findings are in agreement with recent experimental and theoretical works. 1. H. Jonsson, G. Mills and K. W. Jacobsen, in Classical and Quantum Dynamics in Condensed Phase Simulations, ed. by B. J. Berne, G. Ciccotti, and D. F. Coker (World Scientific, Singapore, 1998). 2. L. J. Munro and D. J. Wales, Phys. Rev. B v59, 3969 (1999), and references therein. 3. S. M. Foiles, M. I. Baskes, and M. S. Daw, Phys. Rev. B v33, 7983 (1986).

Trushin, Oleg; Ying, See Chen; Granato, Enzo; Ala-Nissila, Tapio

2001-03-01

289

Spatial and vertical extent of nucleation events in the Midwestern USA: insights from the Nucleation In ForesTs (NIFTy) experiment  

NASA Astrophysics Data System (ADS)

Measurements of aerosol particle physical and chemical properties, gas phase concentrations and meteorological parameters were made along a transect in southern Indiana during the Nucleation In ForesTs (NIFTy) experiment conducted in May 2008. These measurements indicate nucleation was observed at all three measurement sites on almost half of all sampling days. The intensity of the nucleation events, as measured by the increase in ?10 nm aerosol particle number concentrations of approximately 2×104 cm-3 over a layer of at least 300 m depth, is in good agreement with recent model results for the Midwestern USA derived using PMCAMx-UF. During the hour after termination of nucleation approximately half of the number concentration reduction is due to coagulation, while the remainder is due in equal parts to dry deposition and entrainment of relatively ultra-fine aerosol particle free troposphere air. Clear nucleation with continuous subsequent growth is only observed on days when the morning fractional cloud cover was less than 30%. It is associated with a clear transition from a strongly stratified atmosphere with low turbulence intensity and weak vertical velocities, to much a weaker vertical gradient of wind speed, increased turbulence intensity and stronger downwards vertical velocities, consistent with growth of the mixed layer and entrainment of air from the residual layer. Nucleation intensity is not very strongly determined by the prevailing condensational sink. However, there is a strong correlation between both a modified version of the Nucleation Parameter from Boy and Kulmala (2002) and ultrafine aerosol particle number concentrations, and mean morning H2SO4 concentrations and ultrafine aerosol particle number concentrations. Five A-class event days during NIFTy were characterized by values of the dimensionless nucleation parameter of Kuang et al. (2010) that are below 0.3, further indicating the applicability of their postulate that nucleation is favored by L? values below 0.7. Based on aerosol particle composition measurements it appears that aerosol particle formation and initial growth to approximately 30 nm diameter is dominated by ammonium and sulfate. Conservative estimates of the percent contribution of H2SO4 to aerosol particle growth (for sub-30 nm aerosol particles) on five A-class event days ranged from 23 to 85%.

Pryor, S. C.; Barthelmie, R. J.; Sørensen, L. L.; McGrath, J. G.; Hopke, P.; Petäjä, T.

2011-02-01

290

Spatial and vertical extent of nucleation events in the Midwestern USA: insights from the Nucleation In ForesTs (NIFTy) experiment  

NASA Astrophysics Data System (ADS)

Measurements of aerosol particle physical and chemical properties, gas phase concentrations and meteorological parameters were made along a transect in Southern Indiana during the Nucleation In ForesTs (NIFTy) experiment conducted in May 2008. These measurements indicate nucleation was observed at all three measurement sites on almost half of all sampling days. The intensity of the nucleation events, as measured by the increase in ?10 nm aerosol particle number concentrations of approximately 2×104 cm-3 over a layer of at least 300 m depth, is in good agreement with recent model results for the Midwestern USA derived using PMCAMx-UF. During the hour after termination of nucleation approximately half of the number concentration reduction is due to coagulation, while the remainder is due in equal parts to dry deposition and entrainment of relatively ultra-fine aerosol particle free troposphere air. Clear nucleation with continuous subsequent growth is only observed on days when the morning fractional cloud cover was less than 30%, and is associated with a clear transition from a strongly stratified atmosphere with low turbulence intensity and weak vertical velocities, to much a weaker vertical gradient of wind speed, increased turbulence intensity and stronger downwards vertical velocities, consistent with growth of the mixed layer and entrainment of air from the residual layer. Nucleation intensity is not very strongly determined by the prevailing condensational sink. However, there is a strong correlation between both a modified version of the Nucleation Parameter from Boy and Kulmala (2002) and ultrafine aerosol particle number concentrations, and mean morning H2SO4 concentrations and ultrafine aerosol particle number concentrations. Five A-class event days during NIFTy were characterized by values of the dimensionless nucleation parameter of Kuang et al. (2010) that are below 0.3, further indicating the applicability of their postulate that nucleation is favored by L? values below 0.7. Based on aerosol particle composition measurements it appears that aerosol particle formation and initial growth to approximately 30 nm diameter is dominated by ammonium and sulfate. Conservative estimates of the percent contribution of H2SO4 to aerosol particle growth (for sub-30 nm aerosol particles) on five A-class event days ranged from 23 to 85%.

Pryor, S. C.; Barthelmie, R. J.; Sørensen, L. L.; McGrath, J. G.; Hopke, P.; Petäjä, T.

2010-10-01

291

Nucleation driven by time dependent diffusive heat and mass transfer  

NASA Astrophysics Data System (ADS)

Nucleation of fresh aerosol particles in the atmosphere is driven by the prevailing vapor supersaturation and temperature, which often vary with time. We consider the diffusion of heat and vapor into a parcel of gas and the resulting potential for particle nucleation. We focus on the case where a nucleation burst occurs which is self-terminated by the condensation of vapor onto the particles created. Simple consideration of the timescales for the increase and decrease of the nucleation rate at various points within the parcel is sufficient to characterize the productivity of the burst, in agreement with more elaborate numerical simulations. .

Ford, Ian; Barrett, Jonathan; McLean, Michael

2000-08-01

292

Nucleation and growth of combustion flame deposited diamond coatings on silicon nitride  

NASA Astrophysics Data System (ADS)

An investigation has been performed on the kinetics of diamond nucleation on silicon nitride (Si3N4) based materials during oxy-acetylene combustion flame chemical vapor deposition. The deposits were characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) techniques. Kinetic parameters of the nucleation process, such as nucleation rate (I), period of initial nuclei formation (tng), and maximum nucleation density (Nd) were experimentally determined. It was concluded that at low temperatures (Ts < 875°C), the observed nucleation density is due to epitaxial or pseudo-epitaxial growth on residual particles on the substrate surface. At higher temperatures (Ts > 875°C), heterogeneous nucleation of diamond on Si3N4 occurs with an apparent activation energy ( Eahet ) of ˜ 18 kcal/mol. From an Arrhenius plot of particle growth rate, the activation energy (Ea) for diamond growth was calculated to be ˜ 9 kcal/mol. These results suggest that the energy barrier associated with the heterogeneous nucleation process ( Eahet > Ea) may in fact be responsible for the observed low nucleation densities on Si3N4 substrates. Consequently, nucleation density on Si3N4 is limited by the concentration of available sites for nuclei formation. As surface diffusion to those sites was determined to be negligible, growth of stable nuclei occurs via direct impingement of gas phase species. Based on these conclusions, a multistage deposition technique was developed to deposit continuous diamond coatings on untreated Si3N4 substrates. This two stage technique consisted of (i) an in situ flame pretreatment of the substrate to enhance nucleation through the formation of SiC and (ii) subsequent growth on the flame pretreated surface under optimized parameters. Using a previously developed compression test for brittle coating/substrate systems, the adhesion of multistage deposited coatings was compared to conventionally deposited coatings. Normalized values of adhesion for the multistage coatings were observed to be the same order of magnitude as the conventionally seeded diamond coatings. However, the failure mechanisms of the two coatings were observed to be significantly different. Multistage coating failure was characterized by partial ( ˜ typically 5%) delamination while conventionally seeded coatings were observed to completely disintegrate (100% delamination) at failure. Thus, it was projected that the fracture toughness of the multistage deposited coatings may be superior to that of conventionally seeded coatings. Finally, a modified flame technique is introduced to remedy a number of deficiencies and increase the commercial viability of combustion flame synthesis. This technique consists of simple modifications to conventional apparatus which significantly enhances the deposition area and expands the number of candidate substrate materials and geometries that can be coated using combustion flame CVD.

Rozbicki, Robert T.

293

Effect of HCl addition on gas-phase and surface reactions during homoepitaxial growth of SiC at low temperatures  

NASA Astrophysics Data System (ADS)

A complex influence of HCl addition on gas-phase and surface reactions during the low-temperature halo-carbon homoepitaxial growth of 4H-SiC was investigated. The addition of HCl was employed to reduce the undesirable effects of homogeneous gas-phase nucleation leading to formation of silicon clusters in the gas phase. It was established that dissociation of silicon clusters by HCl is efficient even at untraditionally low homoepitaxial growth temperature below 1300 °C. The information about the spatial distribution of this dissociation process along the gas flow direction was obtained. It was established that the influence of HCl is more complicated than the simple model suggesting that the enhanced dissociation of silicon clusters in the gas phase leads to an additional supply of silicon species for the epitaxial growth. While the growth rate does significantly increase at least for some HCl flow rates, complex changes in the effective silicon-to-carbon ratio in the growth zone of the reactor indicate that the supply of carbon species may also be enhanced at least at low HCl flow rates. This fact supports the hypothesis that the gas-phase clusters may contain a significant amount of carbon in addition to silicon. Also, the magnitude of the growth rate enhancement was found to be less significant than what was expected from the apparent degree of the cluster dissociation. Evidence of a new mechanism for precursors' depletion are provided. Pronounced changes in the pattern of the polycrystalline deposits at the upstream portion of the hot zone of the reactor were caused by the HCl addition. Premature dissociation of the gas-phase clusters and release of silicon may cause depletion of silicon by vigorous polycrystalline deposition. Depending on the kinetics of the process, the carbon species may also get depleted by the polycrystalline deposition mechanism. This mechanism partially reduces the benefits of the HCl addition unless its influence is minimized.

Melnychuk, G.; Lin, H. D.; Kotamraju, S. P.; Koshka, Y.

2008-09-01

294

AEROSOL NUCLEATION AND GROWTH DURING LAMINAR TUBE FLOW: MAXIMUM SATURATIONS AND NUCLEATION RATES. (R827354C008)  

EPA Science Inventory

An approximate method of estimating the maximum saturation, the nucleation rate, and the total number nucleated per second during the laminar flow of a hot vapour–gas mixture along a tube with cold walls is described. The basis of the approach is that the temperature an...

295

Uptake of gas-phase formaldehyde by aqueous acid surfaces  

SciTech Connect

The uptake of gas-phase formaldehyde, CH{sub 2}O, by aqueous sulfuric and nitric acid droplets has been measured as a function of temperature, acid concentration, and gas-droplet contact time. The results are interpreted in terms of the mass accommodation coefficient for formaldehyde on aqueous surfaces, the Henry`s law constant for formaldehyde in aqueous acid solutions, and the liquid-phase formation kinetics of methylenediol, CH{sub 2}(OH){sub 2}, and protonated formaldehyde, CH{sub 3}O{sup +}. Time-dependent uptake studies under mildly acid solutions where CH{sub 2}(OH){sub 2} and CH{sub 3}O{sup +} formation is slow reveal the existence of a chemisorbed surface complex for CH{sub 2}O at the gas-liquid interface. Uptake studies on nitric acid and mixed sulfuric acid/nitric acid solutions show slightly enhanced uptake relative to sulfuric acid only solutions. This observation has been attributed to variation of formaldehyde solubility (expressed as Setchenow coefficients) and CH{sub 3}O{sup +} equilibrium constant in nitric and sulfuric acid solutions. The implications of these measurements for the aqueous acid chemistry of formaldehyde and its role in atmospheric chemistry are discussed. 46 refs., 7 figs., 2 tabs.

Jayne, J.T.; Worsnop, D.R.; Kolb, C.E. [Aerodyne Research, Inc., Billerica, MA (United States); Swartz, E.; Davidovits, P. [Boston College, Chestnut Hill, MA (United States)

1996-05-09

296

Dewetting through nucleation  

NASA Astrophysics Data System (ADS)

We review some fundamental aspects of nucleation processes occurring in dewetting phenomena. We elaborate on the analogy between dewetting scenarios and nucleation in bulk systems. While this analogy may be pushed quite far in a few systems, fundamental differences arise in many cases of common interest. We present a number of examples and discuss their relevance in real situations. To cite this article: S. Herminghaus, F. Brochard, C. R. Physique 7 (2006).

Herminghaus, Stephan; Brochard, Françoise

2006-11-01

297

Quantum Control of Femtochemistry in the Gas Phase, Liquid Phase and on Surfaces  

NASA Astrophysics Data System (ADS)

By using coherent control techniques we control the behavior of quantum systems on their natural fs-time scale by applying ultrashort coherent light fields in the wavelength range from the IR to the UV. These laser pulses can be variably shaped in space and time using a laser pulse shaper consisting of a liquid-crystal display [1]. Laser-optimized femtochemistry in the gas phase and liquid phase is one field in which this new technique is successfully employed. Automated optimization of branching ratios and total product yields of gas phase photodissociation reactions as well as chemically selective molecular excitation in the liquid phase is performed [2][3]. Structural changes of a molecule in the liquid phase have been controlled by laser-optimized photoisomerization of a cyanine dye molecule [4] and of retinal in bacteriorhodopsin [5]. So far, optimal control techniques have been restricted to gas phase and condensed phase optimization experiments. Recently we have demonstrated femtosecond laser-assisted catalytic reactions on a Pd(100) single crystal surface. By applying a closed-loop optimal control scheme, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage. The results represent a first step towards selective photocatalysis of molecules. [1] T. Baumert et al, Appl. Phys. B 65, 779 (1997) [2] A. Assion et al, Science 282, 919(1998); T. Brixner et al, J. Mod. Opt. 50, 539 (2003) [3] T. Brixner et al, Nature, Vol. 414, 57 (2001) and J. Chem. Phys. 118, 3692 (2003) [4] G. Krampert et al, Phys. Rev. Lett. 94, 068305 (2005) [5] G. Vogt et al, Chem. Phys. Lett. 433, 211 (2006) P. Nuernberger et al, Phys. Chem. Chem. Phys. 9, 2470 (2007)

Gerber, Gustav

2008-03-01

298

Off-gas recycle for long-term low temperature gas phase uranium decontamination  

SciTech Connect

In situ long-term low-temperature (LTLT) gas phase decontamination is being developed and demonstrated at the K-25 site as a technology that has the potential to substantially lower these costs while reducing criticality and safeguards concerns and worker exposure to hazardous and radioactive materials. The objective of gas phase decontamination is to employ a gaseous reagent to fluorinate nonvolatile uranium deposits to form volatile UF{sub 6}, which can be recovered by chemical trapping or freezing. The LTLT process permits the decontamination of the inside of gas-tight GDP process equipment at room temperature by substituting a long exposure to subatmospheric ClF{sub 3} for higher reaction rates at higher temperatures. Laboratory-scale experiments have demonstrated the feasibility of using LTLT gas phase decontamination with ClF{sub 3} to remove uranium deposits from this equipment. A mobile gas phase system is being designed to demonstrate the decontamination process on a full scale. If used to decontaminate the GDPs, the LTLT process would use large amounts of ClF{sub 3} and exhaust large volumes of by-product gases (ClF, C1O{sub 2}F, etc.). Initially, the excess ClF{sub 3} and reaction byproducts will be destroyed in a KOH scrubber. This paper describes a proposed system that could recover the excess ClF{sub 3}and regenerate the reaction by-products into ClF{sub 3} for use in decontamination of additional equipment. Use of this regeneration and recovery system would reduce raw material costs and also reduce the waste scrubber sludge disposal costs by reducing the amount of corrosive gases fed to the scrubber.

Bundy, R.D.; Bunch, D.H.; Munday, E.B.; Simmons, D.W.

1994-07-01

299

Correlation between gas-phase and solution-phase reactivities of hydroxyl radicals toward saturated organic compounds  

Microsoft Academic Search

The gas-phase and aqueous-solution-phase reactivities of hydroxyl radicals with a wide variety of organic compounds are compared. When kinetic data are available for the same reaction occurring in both phases, this comparison provides useful information about the reaction mechanism. Through this comparison the authors can demonstrate a linear correlation between the gas\\/solution-phase OH reactivities for numerous saturated organic compounds. This

Timothy J. Wallington; Philippe Dagaut; Michael J. Kurylo

1988-01-01

300

Quark Matter Nucleation in Neutron Stars  

NASA Astrophysics Data System (ADS)

We study the quark deconfinement phase transition in cold (T = 0) and hot ?-stable hadronic matter. Assuming a first-order phase transition, we calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) is metastable to the conversion to a quark star (QS) (i.e. hybrid star or strange star). We introduce the concept of critical mass Mcr for cold HSs and proto-hadronic stars, and the concept of limiting conversion temperature for proto-hadronic stars. We show that proto-hadronic stars with a mass M < Mcr could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of proto-hadronic stars.

Bombaci, I.

301

Diffusion in the gas phase: the effects of ambient pressure and gas composition.  

PubMed

Gas transport across the pores of a hen's egg shell occurs by a process of diffusion in the gas phase and for any particular gas depends upon its diffusion coefficient and the pore geometry. The egg shell is thus a convenient model for measuring the diffusive permeability of the shell to a given gas species when its diffusion coefficient is altered by either a change in ambient pressure or by changing the second gas in the diffusion pathway. In this study the permeability of the shell to water vapor and O2 was inversely proportional to ambient pressures over the range of .06 to 8 atmospheres' absolute (ata). The permeability of the shell to water vapor in a He environment (KH20, He) was 2.4 times KH20, air. If KO2, N2 is taken as unity, the permeabilities of the shell to O2 in He, Ar, CO2 and SF6 are 3.38, 0.95, 0.88, and 0.52, respectively. The results are interpreted in terms of the Chapman-Enskog equation, from which binary diffusion coefficients can be predicted for given gas pairs and ambient pressures. These results also provide explantations for the structural modification of egg shells in altitude-adapted chickens, and for the reduced insensible water loss in man at high ambient pressure. PMID:1226462

Paganelli, C V; Rahn, A A; Wangensteen, O D

1975-12-01

302

Printing nanoparticles from the liquid and gas phases using nanoxerography  

NASA Astrophysics Data System (ADS)

This paper reports on the directed self-assembly of nanoparticles onto charged surface areas with a resolution of 200 nm from the liquid phase and 100 nm from the gas phase. The charged areas required for this type of nanoxerographic printing were fabricated using a parallel method that employs a flexible, electrically conductive, electrode to charge a thin-film electret. As electrodes, we used metal-coated polymeric stamps and 10 µm thick doped silicon wafers carrying a pattern in topography. Each electrode was brought in contact with a thin-film electret on an n-doped silicon substrate. The charge pattern was transferred into the thin-film electret by applying a voltage pulse between the conductive electrode and the silicon substrate. Areas as large as 1 cm2 were patterned with charge with 100 nm scale resolution in 10 s. These charge patterns attract nanoparticles. A liquid-phase assembly process where electrostatic forces compete with disordering forces due to ultrasonication has been developed to assemble nanoparticles onto charged based receptors in 10 s from a liquid suspension. A gas-phase assembly process was developed that uses a transparent particle assembly module to direct particles towards the charged surface while monitoring the total charge of assembled particles. Nanoparticles were generated using a tube furnace by evaporation and condensation at the outlet. The electrostatically directed assembly of 10-100 nm sized metal (gold, silver) and 30 nm sized carbon particles was accomplished with a resolution 500-1000 times greater than the resolution of existing xerographic printers.

Barry, Chad R.; Steward, Michael G.; Lwin, Nyein Z.; Jacobs, Heiko O.

2003-10-01

303

Comparing molecular photofragmentation dynamics in the gas and liquid phases.  

PubMed

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement. PMID:23552482

Harris, Stephanie J; Murdock, Daniel; Zhang, Yuyuan; Oliver, Thomas A A; Grubb, Michael P; Orr-Ewing, Andrew J; Greetham, Gregory M; Clark, Ian P; Towrie, Michael; Bradforth, Stephen E; Ashfold, Michael N R

2013-04-04

304

Lattice Gas Model for Immiscible Two Phases with Different Densities  

NASA Astrophysics Data System (ADS)

We developed a new LGA model that has the applicability for two-phase flow with widely different densities. We used the new model based on Rothman and Keller's immiscible lattice gas (ILG) model, and introduced long range interaction to handle widely different densities. Our model has four-step updating rules; the collision step, the interaction step, the rearranging step, and the propagation step. Whereas the collision and the propagation steps are the same as those of the normal LGA model, we introduced non-local interparticle attractive and repulsive force with fixed range on the interaction step and two colored type particles are rearranged by ILG like rules on the rearranging step. We performed phase-separation simulations using our model, and as a result, density distribution coincides completely with color distribution. Furthermore, we introduced new wall boundary conditions considering effects of the non-local interactions. We arranged virtual particle distributions outside of the wall boundary. We attempted two-phase flow simulations with introduced wall boundary conditions, and obtained results which look like water-vapor two-phase flow.

Tsumaya, Akira; Ohashi, Hirotada

1997-08-01

305

Investigation into the Flow Behavior of a Single Phase Gas System and a Two Phase Gas/Liquid System in Normal Gravity with Nonuniform Heating from Above.  

National Technical Information Service (NTIS)

The fluid behavior in normal gravity of a single phase gas system and a two phase gas/liquid system in an enclosed circular cylinder heated suddenly and nonuniformly from above was investigated. Flow visualization was used to obtain qualitative data on bo...

P. J. Disimile T. J. Heist

1990-01-01

306

The effect of gas-phase chemistry on aqueous-phase sulfur dioxide oxidation rates  

Microsoft Academic Search

The rates and mechanisms of both gas and liquid phase reactions for the oxidation of sulfur dioxide play an important role in the production of atmospheric acids and aerosol particles. Rhodeet al. (1981) concluded that sulfate production rates were highly non-linear functions of sulfur dioxide emission rates. Their modelling study used an HOx termination mechanism for the HO—SO2 reaction in

William R. Stockwell

1994-01-01

307

The gas-phase alcoholysis of protonated homoleptic alkoxysilanes.  

PubMed

Tetra-alkoxysilanes are common and useful reagents in sol-gel processes and understanding their reactivity is important in the design of new materials. The mechanism of gas-phase reactions that mimic alcoholyis of Si(OMe)(4) (usually known as TMOS) under acidic conditions have been studied by Fourier transform ion cyclotron resonance techniques and density functional calculations at the B3LYP/6-311+G(d,p) level. The proton affinity of TMOS has been estimated at 836.4 kJ mol(-1) and protonation of TMOS gives rise to an ionic species that is best represented as trimethoxysilyl cations associated with a methanol molecule. Protonated TMOS undergoes rapid and sequential substitution of the methoxy groups in the gas-phase upon reaction with alcohols. The calculated energy profile of the reaction indicates that the substitution reaction through an S(N)2 type mechanism may be more favorable than frontal attack at silicon. Furthermore, the sequential substitution reactions are promoted by a mechanism that involves proton shuttle from the most favorable protonation site to the oxygen of the departing group mediated by the neutral reagent molecule. PMID:20530843

Rainone, Raquel; Malaspina, Thaciana; Xavier, Luciano A; Riveros, José M

2010-01-01

308

Gas phase methylation of methyl acetoacetate. Experimental and theoretical study  

NASA Astrophysics Data System (ADS)

Fourier transform ion cyclotron resonance and tandem mass spectrometry, complemented by semiempirical molecular orbital calculations, have been used to study gas phase methylation of methyl acetoacetate as a mixture of the keto form 1 and the enol form 2. The daughter ion spectra of the ion/molecule reaction products were compared with those of model ions generated by electron impact or chemical ionization, in order to determine the site(s) of nucleophilic reaction for the mixture. The data indicate that the site of attachment in the keto form 1 is the keto-carbonyl oxygen. For the enol form 2, no C-methylation occurs in the gas phase; the only product corresponds to O-alkylation. The results derived from D- and 13C-labelled precursors have been used to study the fragmentation mechanisms of model ions a, CH3C+ (OCH3)CH2CO2CH3; and b, CH3COCH2C+(OCH3)2. Experimental results indicate that an irreversible isomerization a --> b occurs under collisional conditions. Unimolecularly both a and b ions eliminate a neutral molecule of ketene but by different pathways. Calculations of charge distributions in 1 and 2 as well as the enthalpies of the neutral and the adduct ions are discussed.

Morizur, J.-P.; Martigny, I.; Taphanel, M.-H.; Tortajada, J.; Geribaldi, S.; Decouzon, M.

1992-04-01

309

Gas phase reaction of sulfur trioxide with water vapor  

SciTech Connect

Sulfur trioxide (SO3) has long been known to react with water to produce sulfuric acid (H2S04). It has been commonly assumed that the gas phase reaction in the Earth`s atmosphere between SO3 and water vapor to produce sulfuric acid vapor is an important step in the production of sulfuric acid aerosol particles. The kinetics of the gas phase reaction of SO3 with water vapor have previously been studied by Castleman and co-workers, Wang et al and Reiner and Arnold. Each of these studies was carried out in a flow reactor, with the first two studies performed at low pressure (1-10 Torr) and the latter from approx. 30 to 260 Torr. Each of these studies measured SO3 decays over a range of H2O vapor levels, obtaining data consistent with interpreting the reaction of gaseous SO3 and H2O as a bimolecular process. It is not clear why previous experimental studies failed to observe a nonlinear dependence of SO3 consumption on water vapor concentration. It is probable that sufficient water dimer exists in much of the Earth`s atmosphere to allow dimer reactions to participate in sulfuric acid vapor formation.

Kolb, C.E.; Molina, M.J.; Jayne, J.T.; Meads, R.F.; Worsnop, D.R.

1994-12-31

310

Gas-Phase Fragmentation Analysis of Nitro-Fatty Acids  

PubMed Central

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for their structural characterization. Fragmentation of nitroalkanes occurred mainly through loss of the NO2? anion or neutral loss of HNO2. The CID of nitroalkenes proceeds via a more complex cyclization, followed by fragmentation to nitrile and aldehyde products. Gas-phase fragmentation of nitroalkene functional groups with additional ? or ? unsaturation occurred through a multiple step cyclization reaction process, leading to 5 and 6 member ring heterocyclic products and carbon chain fragmentation. Cyclization products were not obtained during nitroalkane fragmentation, highlighting the role of double bond ? electrons during NO2? rearrangements, stabilization and heterocycle formation. The proposed structures, mechanisms and products of fragmentation are supported by analysis of 13C and 15N labeled parent molecules, 6 different nitroalkene positional isomers, 6 nitroalkane positional isomers, accurate mass determinations at high resolution and quantum mechanics calculations. Multiple key diagnostic ion fragments were obtained through this analysis, allowing for the precise placement of double bonds and sites of fatty acid nitration, thus supporting an ability to predict nitro positions in biological samples.

Bonacci, Gustavo; Asciutto, Eliana K.; Woodcock, Steven R.; Salvatore, Sonia R.; Freeman, Bruce A.; Schopfer, Francisco J.

2012-01-01

311

Gas-Phase Fragmentation Analysis of Nitro-Fatty Acids  

NASA Astrophysics Data System (ADS)

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for their structural characterization. Fragmentation of nitroalkanes occurred mainly through loss of the NO{2/-} anion or neutral loss of HNO2. The CID of nitroalkenes proceeds via a more complex cyclization, followed by fragmentation to nitrile and aldehyde products. Gas-phase fragmentation of nitroalkene functional groups with additional ? or ? unsaturation occurred through a multiple step cyclization reaction process, leading to 5 and 6 member ring heterocyclic products and carbon chain fragmentation. Cyclization products were not obtained during nitroalkane fragmentation, highlighting the role of double bond ? electrons during NO{2/-} rearrangements, stabilization and heterocycle formation. The proposed structures, mechanisms and products of fragmentation are supported by analysis of 13C and 15N labeled parent molecules, 6 different nitroalkene positional isomers, 6 nitroalkane positional isomers, accurate mass determinations at high resolution and quantum mechanics calculations. Multiple key diagnostic ion fragments were obtained through this analysis, allowing for the precise placement of double bonds and sites of fatty acid nitration, thus supporting an ability to predict nitro positions in biological samples.

Bonacci, Gustavo; Asciutto, Eliana K.; Woodcock, Steven R.; Salvatore, Sonia R.; Freeman, Bruce A.; Schopfer, Francisco J.

2011-09-01

312

Gas-phase fragmentation analysis of nitro-fatty acids.  

PubMed

Nitro-fatty acids are electrophilic signaling mediators formed in increased amounts during inflammation by nitric oxide and nitrite-dependent redox reactions. A more rigorous characterization of endogenously-generated species requires additional understanding of their gas-phase induced fragmentation. Thus, collision induced dissociation (CID) of nitroalkane and nitroalkene groups in fatty acids were studied in the negative ion mode to provide mass spectrometric tools for their structural characterization. Fragmentation of nitroalkanes occurred mainly through loss of the NO(2)(-) anion or neutral loss of HNO(2). The CID of nitroalkenes proceeds via a more complex cyclization, followed by fragmentation to nitrile and aldehyde products. Gas-phase fragmentation of nitroalkene functional groups with additional ? or ? unsaturation occurred through a multiple step cyclization reaction process, leading to 5 and 6 member ring heterocyclic products and carbon chain fragmentation. Cyclization products were not obtained during nitroalkane fragmentation, highlighting the role of double bond ? electrons during NO(2)(-) rearrangements, stabilization and heterocycle formation. The proposed structures, mechanisms and products of fragmentation are supported by analysis of (13)C and (15)N labeled parent molecules, 6 different nitroalkene positional isomers, 6 nitroalkane positional isomers, accurate mass determinations at high resolution and quantum mechanics calculations. Multiple key diagnostic ion fragments were obtained through this analysis, allowing for the precise placement of double bonds and sites of fatty acid nitration, thus supporting an ability to predict nitro positions in biological samples. PMID:21953257

Bonacci, Gustavo; Asciutto, Eliana K; Woodcock, Steven R; Salvatore, Sonia R; Freeman, Bruce A; Schopfer, Francisco J

2011-07-13

313

Transuranic actinide reactions with simple gas-phase molecules.  

SciTech Connect

The intent of this research is to conduct an experimental study of f-element chemistry fo r the purpose of identifying reaction trends and mechanisms of the early actinide metals with simple gas phase molecules . Previous research has elucidated some of the fundamenta l chemistry of the 4f elements,1-5 however, more complex chemistry is expected for the 5f serie s due to the inclusion of the 5f electrons in the valence shell . The matrix isolation approach, which is well-suited to the experimental study of transient species, will be used for sample collection, and IR/NIR/VIS spectroscopy will be employed to interrogate deposited matrices . The strength of this method lies in the use of isotopes of reactants, which permits the identification of guest molecules in a noble gas matrix by observation of vibrational frequenc y shifts and patterns upon isotopic substitution . Using this technique at the University of Virginia, the first noble gas-actinide bond has recently been identified, a weak U-Ar bond on the CUO molecule.6 Uranium has similarly been observed to bond to krypton and xenon, whereas thoriu m and the lanthanides have not exhibited this activity . It is expected that plutonium will be even more reactive in this respect . We will extend the body of actinide experimental evidence t o include the transuranic elements neptunium, plutonium, and americium reacted with isotopes o f oxygen, nitrogen, hydrogen, carbon monoxide, and carbon dioxide .

Willson, S. P. (Stephen P.); Veirs, D. K. (Douglas Kirk); Baiardo, J. P. (Joseph P.)

2003-01-01

314

Two phase galaxy formation: the gas content of normal galaxies  

NASA Astrophysics Data System (ADS)

We investigate the atomic (HI) and molecular (H2) Hydrogen content of normal galaxies by combining observational studies linking galaxy stellar and gas budgets to their host dark matter (DM) properties, with a physically grounded galaxy formation model. This enables us to analyse empirical relationships between the virial, stellar and gaseous masses of galaxies, and explore their physical origins. Utilizing a semi-analytic model (SAM) to study the evolution of baryonic material within evolving DM haloes, we study the effects of baryonic infall and various star formation and feedback mechanisms on the properties of formed galaxies using the most up-to-date physical recipes. We find that in order to significantly improve the agreement with observations of low-mass galaxies, we must suppress the infall of baryonic material and exploit a two-phase interstellar medium, where the ratio of HI to H2 is determined by the galactic disc structure. Modifying the standard Schmidt-Kennicutt star formation law, which acts upon the total cold gas in galaxy discs and includes a critical density threshold, and employing a star formation law which correlates with the H2 gas mass results in a lower overall star formation rate. This, in turn, allows us to simultaneously reproduce stellar, HI and H2 mass functions of normal galaxies.

Cook, M.; Evoli, C.; Barausse, E.; Granato, G. L.; Lapi, A.

2010-02-01

315

Gas phase chemical studies of superheavy elements using the Dubna gas-filled recoil separator – Stopping range determination  

Microsoft Academic Search

Currently, gas phase chemistry experiments with heaviest elements are usually performed with the gas-jet technique with the disadvantage that all reaction products are collected in a gas-filled thermalisation chamber adjacent to the target. The incorporation of a physical preseparation device between target and collection chamber opens up the perspective to perform new chemical studies. But this approach requires detailed knowledge

D. Wittwer; F. Sh. Abdullin; N. V. Aksenov; Yu. V. Albin; G. A. Bozhikov; S. N. Dmitriev; R. Dressler; R. Eichler; H. W. Gäggeler; R. A. Henderson; S. Hübener; J. M. Kenneally; V. Ya. Lebedev; Yu. V. Lobanov; K. J. Moody; Yu. Ts. Oganessian; O. V. Petrushkin; A. N. Polyakov; D. Piguet; P. Rasmussen; R. N. Sagaidak; A. Serov; I. V. Shirokovsky; D. A. Shaughnessy; S. V. Shishkin; A. M. Sukhov; M. A. Stoyer; N. J. Stoyer; E. E. Tereshatov; Yu. S. Tsyganov; V. K. Utyonkov; G. K. Vostokin; M. Wegrzecki; P. A. Wilk

2010-01-01

316

Feasibility of gas-phase decontamination of gaseous diffusion equipment  

SciTech Connect

The five buildings at the K-25 Site formerly involved in the gaseous diffusion process contain 5000 gaseous diffusion stages as well as support facilities that are internally contaminated with uranium deposits. The gaseous diffusion facilities located at the Portsmouth Gaseous Diffusion Plant and the Paducah Gaseous Diffusion Plant also contain similar equipment and will eventually close. The decontamination of these facilities will require the most cost-effective technology consistent with the criticality, health physics, industrial hygiene, and environmental concerns; the technology must keep exposures to hazardous substances to levels as low as reasonably achievable (ALARA). This report documents recent laboratory experiments that were conducted to determine the feasibility of gas-phase decontamination of the internal surfaces of the gaseous diffusion equipment that is contaminated with uranium deposits. A gaseous fluorinating agent is used to fluorinate the solid uranium deposits to gaseous uranium hexafluoride (UF{sub 6}), which can be recovered by chemical trapping or freezing. The lab results regarding the feasibility of the gas-phase process are encouraging. These results especially showed promise for a novel decontamination approach called the long-term, low-temperature (LTLT) process. In the LTLT process: The equipment is rendered leak tight, evacuated, leak tested, and pretreated, charged with chlorine trifluoride (ClF{sub 3}) to subatmospheric pressure, left for an extended period, possibly > 4 months, while processing other items. Then the UF{sub 6} and other gases are evacuated. The UF{sub 6} is recovered by chemical trapping. The lab results demonstrated that ClF{sub 3} gas at subatmospheric pressure and at {approx} 75{degree}F is capable of volatilizing heavy deposits of uranyl fluoride from copper metal surfaces sufficiently that the remaining radioactive emissions are below limits.

Munday, E.B.; Simmons, D.W.

1993-02-01

317

Spectroscopic studies of molecular iodine emitted into the gas phase by seaweed  

NASA Astrophysics Data System (ADS)

Time profiles of molecular iodine emissions from seven species of seaweed have been measured at high time resolution (7.5 s) by direct spectroscopic quantification of the gas phase I2 using broadband cavity enhanced absorption spectroscopy. Substantial differences were found between species, both in the amounts of I2 emitted when the plants were exposed to air and in the shapes of their emission time profiles. Two species of kelp, Laminaria digitata and Laminaria hyperborea, were found to be the most potent emitters, producing an intense burst of I2 when first exposed to air. I2 was also observed from Saccharina latissima and Ascophyllum nodosum but in lower amounts and with broader time profiles. I2 mixing ratios from two Fucus species and Dictyopteris membranacea were at or below the detection limit of the present instrument (25 pptv). A further set of experiments investigated the time dependence of I2 emissions and aerosol particle formation when fragments of L. digitata were exposed to desiccation in air, to ozone and to oligoguluronate stress factors. Particle formation occurred in all L. digitata stress experiments where ozone and light were present, subject to the I2 mixing ratios being above certain threshold amounts. Moreover, the particle number concentrations closely tracked variations in the I2 mixing ratios, confirming the results of previous studies that the condensable particle-forming gases derive from the photochemical oxidation of the plant's I2 emissions. This work also supports the theory that particle nucleation in the coastal atmosphere occurs in "hot-spot" regions of locally elevated concentrations of condensable gases: the greatest atmospheric concentrations of I2 and hence of condensable iodine oxides are likely to be above plants of the most efficiently emitting kelp species and localised in time to shortly after these seaweeds are uncovered by a receding tide.

Ball, S. M.; Hollingsworth, A. M.; Humbles, J.; Leblanc, C.; Potin, P.; McFiggans, G.

2009-12-01

318

Spectroscopic studies of molecular iodine emitted into the gas phase by seaweed  

NASA Astrophysics Data System (ADS)

Time profiles of molecular iodine emissions from seven species of seaweed have been measured at high time resolution (7.5 s) by direct spectroscopic quantification of the gas phase I2 using broadband cavity enhanced absorption spectroscopy. Substantial differences were found between species, both in the amounts of I2 emitted when the plants were exposed to air and in the shapes of their emission time profiles. Two species of kelp, Laminaria digitata and Laminaria hyperborea, were found to be the most potent emitters, producing an intense burst of I2 when first exposed to air. I2 was also observed from Saccharina latissima and Ascophyllum nodosum but in lower amounts and with broader time profiles. I2 mixing ratios from two Fucus species and Dictyopteris membranacea were at or below the detection limit of the present instrument (25 pptv). A further set of experiments investigated the time dependence of I2 emissions and aerosol particle formation when fragments of L. digitata were exposed to desiccation in air, to ozone and to oligoguluronate stress factors. Particle formation occurred in all L. digitata stress experiments where ozone and light were present, subject to the I2 mixing ratios being above certain threshold amounts. Moreover, the particle number concentrations closely tracked variations in the I2 mixing ratios, confirming the results of previous studies that the condensable particle-forming gases derive from the photochemical oxidation of the plant's I2 emissions. This work also supports the theory that particle nucleation in the coastal atmosphere occurs in "hot-spot" regions of locally elevated concentrations of condensable gases: the greatest atmospheric concentrations of I2 and hence of condensable iodine oxides are likely to be above plants of the most efficiently emitting kelp species and localised in time to shortly after these seaweeds are uncovered by a receding tide.

Ball, S. M.; Hollingsworth, A. M.; Humbles, J.; Leblanc, C.; Potin, P.; McFiggans, G.

2010-07-01

319

Temperature dependence of nucleation in Yukawa fluids  

NASA Astrophysics Data System (ADS)

We have studied the temperature dependence of gas-liquid nucleation in Yukawa fluids with gradient theory (GT) and density functional theory (DFT). Each of these nonclassical theories exhibits a weaker (i.e. better) temperature dependence than classical nucleation theory. At a given temperature, the difference between GT and DFT for the reversible work to form a critical nucleus gets smaller with increasing superaturation. For the temperature dependence, the reversible work for GT is very close to that for DFT at high temperatures. The difference between the two theories increases with decreasing temperature and supersaturation. Thus, in contrast to the behavior of a Peng-Robinson fluid, we find that GT can improve the temperature dependence over that of classical nucleation theory, although not always to the same degree as DFT.

Li, J.-S.; Wilemski, G.

2002-03-01

320

Homogeneous nucleation of methane hydrates: unrealistic under realistic conditions.  

PubMed

Methane hydrates are ice-like inclusion compounds with importance to the oil and natural gas industry, global climate change, and gas transportation and storage. The molecular mechanism by which these compounds form under conditions relevant to industry and nature remains mysterious. To understand the mechanism of methane hydrate nucleation from supersaturated aqueous solutions, we performed simulations at controlled and realistic supersaturation. We found that critical nuclei are extremely large and that homogeneous nucleation rates are extremely low. Our findings suggest that nucleation of methane hydrates under these realistic conditions cannot occur by a homogeneous mechanism. PMID:23148735

Knott, Brandon C; Molinero, Valeria; Doherty, Michael F; Peters, Baron

2012-11-19

321

Gas-Phase Dissociation Pathways of Multiply Charged Peptide Clusters  

PubMed Central

Numerous studies of cluster formation and dissociation have been conducted to determine properties of matter in the transition from the condensed phase to the gas phase using materials as diverse as atomic nuclei, noble gasses, metal clusters, and amino acids. Here, electrospray ionization is used to extend the study of cluster dissociation to peptides including leucine enkephalin with 7–19 monomer units and 2–5 protons, and somatostatin with 5 monomer units and 4 protons under conditions where its intramolecular disulfide bond is either oxidized or reduced. Evaporation of neutral monomers and charge separation by cluster fission are the competing dissociation pathways of both peptides. The dominant fission product for all leucine enkephalin clusters studied is a proton-bound dimer, presumably due to the high gas-phase stability of this species. The branching ratio of the fission and evaporation processes for leucine enkephalin clusters appears to be determined by the value of z2/n for the cluster where z is the charge and n the number of monomer units in the cluster. Clusters with low and high values of z2/n dissociate primarily by evaporation and cluster fission respectively, with a sharp transition between dissociation primarily by evaporation and primarily by fission measured at a z2/n value of ~0.5. The dependence of the dissociation pathway of a cluster on z2/n is similar to the dissociation of atomic nuclei and multiply charged metal clusters indicating that leucine enkephalin peptide clusters exist in a state that is more disordered, and possibly fluid, rather than highly structured in the dissociative transition state. The branching ratio, but not the dissociation pathway of [somatostatin5 + 4H]4+ is altered by the reduction of its internal disulfide bond indicating that monomer conformational flexibility plays a role in peptide cluster dissociation.

Jurchen, John C.; Garcia, David E.; Williams, Evan R.

2005-01-01

322

A numerical investigation into factors affecting gas and aqueous phase plumes in the subsurface  

Microsoft Academic Search

An investigation into the face and transport of volatile organic compounds (VOCs) in the subsurface requires the consideration of contaminant mass in both the aqueous and soil gas phases. As a result of water\\/gas phase partitioning, contaminated by partitioning from underlying ground water pollution. Conversely, soil gas can be contaminated by partitioning from underlying ground water VOC plumes. This soil

N. R. Thomson; J. F. Sykes; D. Van Vliet

1997-01-01

323

Determination of gas phase adsorption isotherms—a simple constant volume method  

Microsoft Academic Search

Single and ternary solute gas phase adsorption isotherms were conducted in this study to evaluate the effectiveness of a simple constant volume method, which was utilized by using Tedlar gas sampling bags as a constant volume batch reactor. For this purpose, gas phase adsorption of toluene, methyl ethyl ketone (MEK), and methyl isobutyl ketone (MIBK) on two types of activated

Daekeun Kim; Zhangli Cai; George A. Sorial

2006-01-01

324

Frequency metrology of a photomixing source for gas phase spectroscopy  

NASA Astrophysics Data System (ADS)

The availability of frequency combs has opened new possibilities for the measurement of optical frequencies. Photomixing is an attractive solution for high resolution THz spectroscopy of gases due to the narrow spectral resolution and ability to access the 100 GHz to 3.5 THz range. One limitation of present photomixing spectrometers is the accuracy with which the THz frequency is established. Measurement of the centre frequency gas phase molecular transitions requires an accuracy better than 100 kHz in order to allow spectroscopic constants to be determined. Standard optical techniques like those employed in wavelength meters can only provide accuracies in the order of 50 MHz. We have used a turnkey fibre based frequency comb and a standard photomixing configuration to realize a THz synthesizer with an accuracy of around 50kHz. Two ECDLs used to pump the photomixer are phase locked onto the frequency comb and provide a tuning range of 10 MHz. In order to extend the tuning range an additional phase locked ECLD has been added to obtain a range in excess of 100 MHz. The absorption profiles of many Doppler limited transitions of carbonyl sulphide and formaldehyde have been measured to validate this instrument.

Hindle, Francis; Mouret, Gael; Yang, Chun; Cuisset, Arnaud; Bocquet, Robin; Lours, Michel; Rovera, Daniele

2010-08-01

325

Dumbbells and onions in ternary nucleation.  

PubMed

Molecular simulations for a ternary nucleation system (water/n-nonane/1-butanol) demonstrate a more complex nucleation mechanism than previously thought, where critical nuclei with different compositions are present even for a given vapour-phase composition; the spatial distribution in these critical nuclei is heterogeneous and dumbbell and onion motifs are found; in the former, water and nonane nano-droplets are connected through a butanol handle, while in the latter a water core is surrounded by a nonane corona with an interfacial butanol shell. PMID:17538724

Nellas, Ricky B; Chen, Bin; Siepmann, J Ilja

2007-05-03

326

Application of Phase-field Method in Predicting Gas Bubble Microstructure Evolution in Nuclear Fuels  

Microsoft Academic Search

Fission product accumulation and gas bubble microstructure evolution in nuclear fuels strongly affect thermo-mechanical properties such as thermal conductivity, gas release, volumetric swelling and cracking, and hence the fuel performance. In this paper, a general phase-field model is developed to predict gas bubble formation and evolution. Important materials processes and thermodynamic properties including the generation of gas atoms and vacancies,

Shenyang Y. Hu; Yulan Li; Xin Sun; Fei Gao; Ramaswami Devanathan; Charles H. Henager Jr.; Mohammad A. Khaleel

2010-01-01

327

Uncertainties in gas-phase atmospheric iodine chemistry  

NASA Astrophysics Data System (ADS)

We present a comprehensive chemical mechanism for gas-phase iodine, to be used for modelling tropospheric chemistry. The mechanism has been compiled from evaluated data and individual literature studies, where available; a number of key processes have not been studied experimentally or theoretically and in these cases estimations have been made. The uncertainty associated with these assumptions is evaluated. We analyze the mechanism using a box-model under a variety of boundary layer scenarios - representative of environments where iodine species have been observed - to study the response of the chemical system to changes in the kinetic parameters of selected reactions. We focus in particular on key species such as IO, OIO, INO3 and I2Oy and the impact of iodine chemistry on ozone formation and HOx levels. The results indicate that the chemical system is most sensitive to reactions leading to comparatively stable iodine compounds, which should be a focus of future laboratory studies.

Sommariva, R.; Bloss, W. J.; von Glasow, R.

2012-09-01

328

The intrinsic (gas-phase) acidities of bridgehead alcohols  

NASA Astrophysics Data System (ADS)

The gas-phase acidities of 1-adamantanol and perfluoro1-adamantanol were determined by means of Fourier transform ion cyclotron resonance spectrometry (FT-ICR). The acidity of perfluoro1-adamantanol seems to be the highest ever reported for an alcohol. A computational study of these species and their anions at both the MP2/6-311 + G(d,p) and B3LYP/6-311 + G(d,p) levels was performed. Also studied were the tertiary alcohols (including their perfluorinated forms) derived from norbornane, bicyclo[2.2E2]octane and cubane. It was found that: (i) the intrinsic acidity of non-fluorinated bridgehead alcohols increases with the strain of the hydrocarbon framework and, (ii) perfluorination of these compounds strongly increases their acidity and, likely, significantly modifies their internal strain.

Herrero, Rebeca; Dávalos, Juan Z.; Abboud, José-Luis M.; Alkorta, I.; Koppel, I.; Koppel, I. A.; Sonoda, T.; Mishima, M.

2007-11-01

329

Ceramic microreactors for heterogeneously catalysed gas-phase reactions.  

PubMed

The high surface to volume ratio of microchannel components offers many advantages in micro chemical engineering. It is obvious, however, that the reactor material and corrosion phenomena play an important role when applying these components. For chemical reactions at very high temperatures or/and with corrosive reactants involved, microchannel components made of metals or polymers are not suited. Hence, a modular microreactor system made of alumina was developed and fabricated using a rapid prototyping process chain. With exchangeable inserts the system can be adapted to the requirements of various reactions. Two heterogeneously catalysed gas-phase reactions (oxidative coupling of methane, isoprene selective oxidation to citraconic anhydride) were investigated to check the suitability of the system at temperatures of up to 1000 degrees C. Apart from the high thermal and chemical resistance, the lack of any blind activity was found to be another advantage of ceramic components. PMID:15269808

Knitter, Regina; Liauw, Marcel A

2004-05-03

330

Gas-phase syntheses for interstellar carboxylic and amino acids  

NASA Astrophysics Data System (ADS)

We report experimental results that demonstrate gas-phase, ionic syntheses of glycine and ?-alanine, as well as acetic and propanoic acid, from smaller molecules found in space; in doing so, we infer the formation of these acids in the interstellar environment. We show that ionized glycine and ?-alanine are produced in the reactions of hydroxylamine ions, NH2OH+, with acetic and propanoic acid respectively. Even more promising in the context of interstellar synthesis are our results that demonstrate the corresponding production of the protonated amino acids from analogous reactions with protonated hydroxylamine. The striking specificity of these syntheses for the ?-isomer of alanine suggests that the amino acids of CI (Carbonaceous Ivona) chondrite meteorites are products of interstellar chemistry and supports the hypothesis that these meteorites are of cometary origin.

Blagojevic, Voislav; Petrie, Simon; Bohme, Diethard K.

2003-02-01

331

Gas-phase spectroscopy of biomolecular ions: Porphyrins and metalloporphyrins  

NASA Astrophysics Data System (ADS)

The electronic structure of a biochromophore (i.e., light absorber) is strongly perturbed by its environment, e.g., water or amino acid residues within protein pockets. To reveal the intrinsic electronic properties, it is therefore necessary to study isolated molecules in vacuo. Many biochromophores are ionic in their natural environment, which renders experiments complicated as it is not possible to produce enough absorbing species for traditional light transmission spectroscopy. In Aarhus we have developed state-of-the-art apparatus to record gas-phase absorption spectra. Some recent results for porphyrin and metalloporphyrin ions are presented, including both electronic ground state ions and electronically excited ions. Fragmentation channels are found from quickly switching the ring voltages after photoexcitation to store particular daughter ions.

Brøndsted Nielsen, Steen

2012-11-01

332

Infrared photodissociation spectroscopy of protonated neurotransmitters in the gas phase  

NASA Astrophysics Data System (ADS)

Protonated neurotransmitters have been produced in the gas phase via a novel photochemical scheme: complexes of the species of interest, 1-phenylethylamine, 2-amino-1-phenylethanol and the diastereo-isomers, ephedrine and pseudoephedrine, with a suitable proton donor, phenol (or indole), are produced in a supersonic expansion and ionized by resonant two photon ionization of the donor. Efficient proton transfer generates the protonated neurotransmitters, complexed to a phenoxy radical. Absorption of infrared radiation, and subsequent evaporation of the phenoxy tag, coupled with time of flight mass spectrometry, provides vibrational spectra of the protonated (and also hydrated) complexes for comparison with the results of quantum chemical computation. Comparison with the conformational structures of the neutral neurotransmitters (established previously) reveals the effect of protonation on their structure. The photochemical proton transfer strategy allows spectra to be recorded from individual laser shots and their quality compares favourably with that obtained using electro-spray or matrix assisted laser desorption ion sources.

MacLeod, N. A.; Simons, J. P.

2007-03-01

333

Synthesis and Gas Phase Thermochemistry of Germanium-Containing Compounds  

SciTech Connect

The driving force behind much of the work in this dissertation was to gain further understanding of the unique olefin to carbene isomerization observed in the thermolysis of 1,1-dimethyl-2-methylenesilacyclobutane by finding new examples of it in other silicon and germanium compounds. This lead to the examination of a novel phenylmethylenesilacyclobut-2-ene, which did not undergo olefin to carbene rearrangement. A synthetic route to methylenegermacyclobutanes was developed, but the methylenegermacyclobutane system exhibited kinetic instability, making the study of the system difficult. In any case the germanium system decomposed through a complex mechanism which may not include olefin to carbene isomerization. However, this work lead to the study of the gas phase thermochemistry of a series of dialkylgermylene precursors in order to better understand the mechanism of the thermal decomposition of dialkylgermylenes. The resulting dialkylgermylenes were found to undergo a reversible intramolecular {beta} C-H insertion mechanism.

Nathan Robert Classen

2002-12-31

334

Structures of platinum oxide clusters in the gas phase.  

PubMed

The structures of small gas-phase Pt(n)O(2m)(+) (n = 1-6, m = 1, 2) cluster cations have been investigated in a combined infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) study. On the basis of the infrared spectra obtained, it is concluded that in most clusters oxygen is bound dissociatively, preferring 2-fold bridge binding motifs, sometimes combined with singly coordinated terminal binding. Comparison of the oxide cluster structures with those of bare cationic platinum clusters reported previously reveals major structural changes induced in the platinum core upon oxygen binding. For some cluster sizes the presence of the Ar messenger atom(s) is found to induce a significant change in the observed cluster structure. PMID:22894744

Kerpal, Christian; Harding, Dan J; Hermes, Alexander C; Meijer, Gerard; Mackenzie, Stuart R; Fielicke, André

2012-08-28

335

Nucleation rate for black holes  

NASA Astrophysics Data System (ADS)

A simple heuristic calculation is given for the rate of nucleation of black holes at positive temperature. This calculation is based on the classical theory of nucleation and reproduces the result of Gross, Perry, and Yaffe.

Kapusta, Joseph I.

1984-08-01

336

Observation of zwitterion formation in the gas-phase H\\/D-exchange with CH 3 OD: Solution-phase structures in the gas phase  

Microsoft Academic Search

Infrared spectroscopy of gas-phase singly deuterated [Trp+K]+ (formed by H\\/D exchange with CH3OD) shows that some (?20%) kinetically stable zwitterionic (ZW) conformer is formed, based on the diagnostic antisymmetric\\u000a CO stretch of the deprotonated carboxylate moiety, ?as(CO2?), at 1680 cm?1. A majority of the deuterated [Trp+K]+ is found to be in the charge solvation (CS) conformation, with deuterium exchange occurring

Nick C. Polfer; Robert C. Dunbar; Jos Oomens

2007-01-01

337

Gas phase chromatography of halides of elements 104 and 105  

SciTech Connect

On-line isothermal gas phase chromatography was used to study halides of {sup 261}104 (T{sub {1/2}} = 65 s) and {sup 262,263}105 (T{sub {1/2}} = 34 s and 27 s) produced an atom-at-a time via the reactions {sup 248}Cm({sup 18}O, 5n) and {sup 249}Bk({sup 18}O, 5n, 4n), respectively. Using HBr and HCl gas as halogenating agents, we were able to produce volatile bromides and chlorides of the above mentioned elements and study their behavior compared to their lighter homologs in Groups 4 or 5 of the periodic table. Element 104 formed more volatile bromides than its homolog Hf. In contrast, element 105 bromides were found to be less volatile than the bromides of the group 5 elements Nb and Ta. Both 104 and Hf chlorides were observed to be more volatile than their respective bromides. 31 refs., 8 figs.

Tuerler, A.; Gregorich, K.E.; Czerwinski, K.R.; Hannink, N.J.; Henderson, R.A.; Hoffman, D.C.; Kacher, C.D.; Kadkhodayan, B.; Kreek, S.A.; Lee, D.M.; Leyba, J.D.; Nurmia, M.J. (Lawrence Berkeley Lab., CA (United States)); Gaeggeler, H.W.; Jost, D.T.; Kovacs, J.; Scherer, U.W.; Vermeulen, D.; Weber, A. (Paul Scherrer Inst. (PSI), Villigen (Switzerland)); Barth, H.; Gober, M.K.; Kratz, J.V. (Philipps-Univ., Marburg

1991-04-01

338

Estimation of gas phase mixing in packed beds  

SciTech Connect

An improved model is presented for estimation of the mixing of gaseous species in a packed bed for fuel conversion. In particular, this work clarifies the main characteristics of mixing of volatiles and oxidizers in a burning bed of high-volatile solid fuel. Expressions are introduced to represent the active role of degradation of the solid particles in the mixing within the gas phase. During drying and devolatilization the solids modify the behavior of the gas flow: the volatiles released from the surface of the particles increase the turbulence in the system, and hence the rates of the homogeneous reactions under mixing-limited conditions. Numerical experiments are carried out to test the validity of this conclusion regarding mixing in different geometries. The flow of volatiles leaving the fuel particles is shown to contribute significantly to mixing, especially at low air flows through a bed. However, the fraction of the particle surface where volatiles are released and its orientation in the bed should be better determined in order to increase the accuracy of the estimates of turbulent mixing. (author)

Frigerio, S. [CMIC Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan (Italy); Thunman, H.; Leckner, B.; Hermansson, S. [Department of Energy Conversion, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)

2008-04-15

339

Liquid chromatographic arsenic speciation with gas-phase chemiluminescence detection.  

PubMed

We present a newly developed gas-phase chemiluminescence (CL) detection method for the separation and quantification of inorganic and organic arsenic species. Arsenite, arsenate, dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA) were separated by anion exchange using carbonate-bicarbonate and NaOH eluents with step-gradient elution. The separated species were passed through a UV photooxidation reactor which decomposed the organic species and converted them to inorganic As(V). Subsequent on-line hydride generation with acid and sodium borohydride produces AsH3 and H2, which are separated from the liquid in a gas-liquid separator. The produced AsH3, driven by H2, reacts with ozone in a small reflective cell located atop a photomultiplier tube, resulting in intense CL. In the present form, the limits of detection (LODs, signal-to-noise = 3), based on peak height, for arsenite, arsenate, MMA, and DMA are 0.4, 0.2, 0.5, and 0.3 microg/L, respectively, for a 100 microL injected sample. This analyzer demonstrates the robustness of the CL detection system for arsenic and provides an affordable alternative to atomic spectrometry for use as a detector after chromatographic speciation. We found no significant practical interferences. PMID:17949013

Idowu, Ademola D; Dasgupta, Purnendu K

2007-10-19

340

Synthesis of carbon nanbotubes by plasma-enhanced CVD process: gas phase study of synthesis conditions  

NASA Astrophysics Data System (ADS)

To support experimental investigations, a model based on Chemkin^TM software was used to simulate gas phase and surface chemistry during plasma-enhanced catalytic CVD of carbon nanotubes. According to these calculations, gas phase composition, etching process and growth rates are calculated. The role of several carbon species, hydrocarbon molecules and ions in the growth mechanism of carbon nanotubes is presented in this study. Study of different conditions of gas phase activation sources and pressure is performed.

Guláš, M.; Cojocaru, C. S.; Fleaca, C. T.; Farhat, S.; Veis, P.; Le Normand, F.

2008-09-01

341

Flow characteristics of gas–liquid two phase plunging jet absorber–gas holdup and bubble penetration depth  

Microsoft Academic Search

A gas-liquid two phase plunging jet is formed through a gas sucking type multi-jet ejector nozzle. In this study, the effects\\u000a of various conditions in the multi-jet ejector nozzle, the column diameter, and the liquid jet length on penetration depth\\u000a of air bubblesl\\u000a \\u000a B\\u000a and gas holdup hG in a gas-liquid two phase plunging jet absorber were studied experimentally. Consequently,

Mitsuharu Ide; Hiroki Uchiyama; Toshifumi Ishikura

1999-01-01

342

Evolution of the Normal State of a Strongly Interacting Fermi Gas from a Pseudogap Phase to a Molecular Bose Gas  

NASA Astrophysics Data System (ADS)

Wave-vector resolved radio frequency spectroscopy data for an ultracold trapped Fermi gas are reported for several couplings at Tc, and extensively analyzed in terms of a pairing-fluctuation theory. We map the evolution of a strongly interacting Fermi gas from the pseudogap phase into a fully gapped molecular Bose gas as a function of the interaction strength, which is marked by a rapid disappearance of a remnant Fermi surface in the single-particle dispersion. We also show that our theory of a pseudogap phase is consistent with a recent experimental observation as well as with quantum Monte Carlo data of thermodynamic quantities of a unitary Fermi gas above Tc.

Perali, A.; Palestini, F.; Pieri, P.; Strinati, G. C.; Stewart, J. T.; Gaebler, J. P.; Drake, T. E.; Jin, D. S.

2011-02-01

343

On the ice nucleation spectrum  

NASA Astrophysics Data System (ADS)

This work presents a novel formulation of the ice nucleation spectrum, i.e. the function relating the ice crystal concentration to cloud formation conditions and aerosol properties. The new formulation relies on a statistical view of the ice nucleation process and explicitly accounts for the dependency of the ice crystal concentration on temperature, supersaturation, cooling rate, and particle size, and, in the case of heterogeneous ice nucleation, on the distributions of particle area and surface composition. The new formulation is used to generate ice nucleation parameterizations for the homogeneous freezing of cloud droplets and the heterogeneous deposition ice nucleation on dust and soot ice nuclei. For homogeneous freezing, it was found that by increasing the dispersion in the droplet volume distribution the fraction of supercooled droplets in the population increases. For heterogeneous ice nucleation it was found that ice nucleation on efficient ice nuclei (IN) shows features consistent with the singular hypothesis (characterized by a lack of temporal dependency of the ice nucleation spectrum) whereas less efficient IN tend to display stochastic behavior. Analysis of empirical nucleation spectra suggested that inferring the aerosol heterogeneous ice nucleation properties from measurements of the onset supersaturation and temperature may carry significant error as the variability in ice nucleation properties within the aerosol population is not accounted for. This work provides a simple and rigorous ice nucleation framework were theoretical predictions, laboratory measurements and field campaign data can be reconciled, and that is suitable for application in atmospheric modeling studies.

Barahona, D.

2011-11-01

344

Atomic and molecular physics in the gas phase.  

PubMed

The spatial and temporal distributions of energy deposition by high-linear-energy-transfer radiation play an important role in the subsequent chemical and biological processes leading to radiation damage. Because the spatial structures of energy deposition events are of the same dimensions as molecular structures in the mammalian cell, direct measurements of energy deposition distributions appropriate to radiation biology are infeasible. This circumstance has led to the development of models of energy transport based on a knowledge of atomic and molecular interactions that enable one to simulate energy transfer on an atomic scale. Such models require a detailed understanding of the interactions of ions and electrons with biologically relevant material. During the past 20 years, there has been a great deal of progress in our understanding of these interactions, much of it coming from studies in the gas phase. These studies provide information on the systematics of interaction cross sections, and lead to knowledge of the regions of energy deposition where molecular and phase effects are important-knowledge that guides development in appropriate theory. In this report, studies of the doubly differential cross sections, which are crucial to the development of stochastic energy deposition calculations and track structure simulation, are reviewed. We discuss areas of understanding and address directions for future work. Particular attention is given to experimental and theoretical findings that have changed the traditional view of secondary electron production for charged-particle interactions with atomic and molecular targets. PMID:1811483

Toburen, L H

1991-01-01

345

Homogeneous nucleation rate measurements in supersaturated water vapor  

NASA Astrophysics Data System (ADS)

The rate of homogeneous nucleation in supersaturated vapors of water was studied experimentally using a thermal diffusion cloud chamber. Helium was used as a carrier gas. Our study covers a range of nucleation rates from 3×10-1 to 3×102 cm-3 s-1 at four isotherms: 290, 300, 310, and 320 K. The molecular content of critical clusters was estimated from the slopes of experimental data. The measured isothermal dependencies of nucleation rate of water on saturation ratio were compared with the prediction of the classical theory of homogeneous nucleation, the empirical prediction of Wölk et al. [J. Chem. Phys. 117, 10 (2002)], the scaled model of Hale [Phys. Rev. A 33, 4156 (1986)], and the former nucleation onset data.

Brus, David; Ždímal, Vladimír; Smolík, Ji?í

2008-11-01

346

Nucleation theorems, the statistical mechanics of molecular clusters, and a revision of classical nucleation theory  

NASA Astrophysics Data System (ADS)

The nucleation theorems relate the temperature and supersaturation dependence of the rate of nucleation of droplets from a metastable vapor phase to properties of the critical molecular cluster, the size that is approximately equally likely to grow or decay. They are derived here using a combination of statistical mechanics and cluster population dynamics, using an arbitrary model cluster definition. The theorems are employed to test the validity of the classical theory of homogeneous nucleation and its ``internally consistent'' form. It is found that the properties of the critical cluster for these models are incorrect, and it emerges that this occurs because the classical theory employs the free energy of a fixed droplet, rather than one free to take any position in space. Thus a term representing positional, or mixing, entropy is missing from the cluster free energy. A revised model is proposed, based on the capillarity approximation but with such a term included, and it is shown that it is fully consistent with the nucleation theorems. The model increases classical rates by factors of approximately 104-106. Other nucleation models should be tested for internal consistency using the same methods. Finally, the nucleation theorems are used to extract the excess internal energies of molecular clusters from experimental data for several substances.

Ford, I. J.

1997-11-01

347

Selective and sensitive chromogenic detection of cyanide and HCN in solution and in gas phase.  

PubMed

Two triphenylmethane based chemodosimeters for selective and chromogenic sensing of cyanide anions in aqueous environments and of hydrogen cyanide in gas phase were prepared and studied. PMID:23680816

Gotor, Raúl; Costero, Ana M; Gil, Salvador; Parra, Margarita; Martínez-Máñez, Ramón; Sancenón, Félix; Gaviña, Pablo

2013-05-17

348

Gas-phase nuclear magnetic resonance studies of conformational kinetics  

NASA Astrophysics Data System (ADS)

The gas-phase conformational exchange kinetics of several amide and thioamide systems have been examined. The most fundamental amide system, formamide, has been studied extensively. The internal rotation rate constants for gaseous formamide are temperature and pressure dependent. The temperature dependent unimolecular high-pressure limit activation parameters are consistent with E? = 16.6(0.3) kcal mol-1 and A ? = 1.60(1.02) × 1013 sec-1 . The pressure dependent rate constants for formamide internal rotation are found to follow statistical RRKM kinetics. In addition, experimental and computational conformational analyses have been performed for solvated formamide systems. The internal rotation activation barriers are found to increase with increasing polarity of the solvent. The experimental values are compared to values calculated according to SC - IPCM and IPCM methodologies. The conformational exchange and equilibria in gas-phase asymmetrically substituted amides and thioamides have also been studied. The internal rotation activation barriers and conformational equilibria are found to be primarily influenced by the nature of the carbonyl substituent on the carbonyl or thiocarbonyl carbon in amides and thioamides. The conformational equilibria and activation energies are significantly different for the thioamides relative to their oxoamide analogs. Solvation significantly affects both the activation and conformational energies for all systems. The phase dependence of the values is attributed to solvent internal pressure effects and solvent stabilizing interactions. The Berry pseudorotation process in gaseous SF4 has been examined. Rate constants determined from exchange broadened 19F NMR spectra are temperature and pressure dependent. The temperature dependence of the unimolecular rate constants are consistent with E? = 11.9(0.2) kcal mol-1, A? = 3.56(1.09) × 1012 sec-1, ?G? = 12.2(0.2) kcal mol-1, ?H? = 11.3(0.4) kcal mol-1, and ?S? = -3.3(0.4) cal mol-1 K-1. The experimental values are compared to theoretical predictions using HF, MP2, DFT, MP4, and G2 methods. The pressure dependent rate constants agree well with RRKM predictions. Finally, the degenerate migration of the trimethylsilyl group in gaseous 5-trimethylsilylcyclopentadiene is studied. The temperature dependent rate constants for this process are consistent with ?G? = 15.7(0.1) kcal mol-1, ?H? = 15.4(0.2) kcal mol-1, and ?S? = -1.1(0.1) cal mol-1 K-1. The kinetic parameters are slightly higher than previously reported values for the neat liquid.

Taha, Angela Najah

349

The electronic structure of gas phase croconic acid compared to the condensed phase: More insight into the hydrogen bond interaction  

NASA Astrophysics Data System (ADS)

The electronic structure of croconic acid in the gas phase has been investigated by means of core level and valence band photoemission spectroscopy and compared with hybrid Heyd-Scuseria-Ernzerhof density functional theory calculations. The results have been compared with the corresponding ones of the condensed phase. In the gas phase, due to the absence of hydrogen bond intermolecular interactions, the O 1 s core level spectrum shows a shift of binding energy between the hydroxyl (O-H) and the carbonyl group (C=O) of 2.1 eV, which is larger than the condensed phase value of 1.6 eV. Interestingly, such a shift decreases exponentially with the increase of the O-H distance calculated from theory. The significant differences between the gas and condensed phase valence band spectra highlight the important role played by the hydrogen bonding in shaping the electronic structure of the condensed phase.

Bisti, F.; Stroppa, A.; Perrozzi, F.; Donarelli, M.; Picozzi, S.; Coreno, M.; de Simone, M.; Prince, K. C.; Ottaviano, L.

2013-01-01

350

Quantum control of gas-phase and liquid-phase femtochemistry.  

PubMed

Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum-mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user-defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so-called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas-phase as well as liquid-phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer-guided optimization methods. The general approach to active control of light-matter interaction has also applications in many other areas of modern physics and related disciplines. PMID:12785256

Brixner, Tobias; Gerber, Gustav

2003-04-14

351

Gas phase processing of nanostructured titania particles and films: Environmentally benign methodologies  

NASA Astrophysics Data System (ADS)

This dissertation is a study of the gas phase synthesis using flame aerosol reactors to produce nanostructured particles and films with desired features as size, morphology, agglomeration state, crystalline phase composition and modified solid state properties. The following particles/films were produced: titania, lead titanate, Mn-Zn ferrite, Fesp{3+} doped titania and nanocomposites of FeO/TiO. On supercooling titania droplets in flames, metastable anatase was preferentially nucleated. At high flame temperatures of 1570sp°C, 100% anatase spherical titania particles were obtained; at 900 ˜ 1430sp°C, polyhedral shaped particles with mixtures of anatase and rutile were obtained; and at 400sp°C, amorphous titania was obtained. A 3-D computer simulation was used to study the aggregration and restructuring of small clusters in flames. These small aggregates are fractal-like and comply with the fractal power law only in a statistical sense, with a decrease in fractal dimension as the clusters become smaller. A cluster-restructuring model was developed to simulate the topological evolution in a high temperature sintering process. The fractal dimension was found to increase as sintering proceeded for small clusters, in contrast to large clusters where the fractal dimension remained constant. A light scattering technique was used to capture the sintering restructuring of nanosized titania particles in flames by in situ determination of the fractal dimension change of titania agglomerates. Laser Induced Fluorescence was used to optimize flame conditions to produce perovskite structured lead titanate particles in flame reactors. Also, the flame reaction provides a novel method to synthesize Fe(III) doped titania. As the doping concentrations varied from trace to comparable to TiOsb2, solid state properties of titania were modified. Substitutionally doped ferric ion accelerated the anatase-rutile transformation in flames, resulting in a more compact lattice structure and generating oxygen deficiency defects, as revealed by the increased shifting and broadening of anatase peaks in the Raman spectra. Further the UV/VIS absorption spectra was observed to be shifted towards the visible. As the iron dopant increased to the amount of comparable with titanium, the as produced pseudo binary mixture of FeO/TiO had superparamagnetic characteristics indicated by Mossbauer and magnetization measurements. Flame assisted aerosol coating processes were used to deposited titania films onto stainless steel and silica substrates. Deposition mechanisms were studied and related to the film characteristics. Interesting solid state properties were observed as the film thickness diminished to nanoscales. The titania films were tested to have a good anti-corrosive characteristics and other exciting functionality having wide applications.

Yang, Guixiang

352

The photochemical reaction of excited acetophenone and benzaldehyde in the gas phase  

NASA Astrophysics Data System (ADS)

The photochemistry of acetophenone and benzaldehyde with hydrogen donors has been studied in the gas phase. The quenching rate constants of the triplet acetophenone and benzaldehyde can be well correlated with the ionization potentials of hydrogen donors. The results suggested that the charge-transfer interaction plays an important role in the gas-phase reaction as in the case of condensed phase reaction. The slopes of Rehm-Weller plot are more stiff in the gas-phase reaction than condensed phase reaction, which indicates higher degree of charge transfer. Secondary amines were consistently better quencher of carbonyl triplet than tertiary amines, and steric effects are probably to be of importance.

Matsushita, Yoshihisa; Yamaguchi, Yoshitaka; Hikida, Takumi

1996-12-01

353

Simulation of the transfer of water with a fine-disperse gas phase in porous media  

NASA Astrophysics Data System (ADS)

The problem on the transfer of a three-phase water-gas-oil mixture in a porous medium was solved for the case where the water contains a fine-disperse gas phase in the form of microsized or nanosized bubbles. It was suggested that the transfer of bubbles is mainly due to the flow of the disperse phase (water). In this case, the large aggregates of the gas phase in the porous space, in the water, and in the oil are transferred in accordance with the modified Darcy law for multiphase mixtures. A mathematical model of movement of the indicated mixture has been constructed for the case where the main phases (water, gas, and oil) adhere to the filtration equations and the fine-disperse gas phase is defined by a kinetic equation like the Boltzmann equation. Some one-dimensional numerical solutions of the indicated problem were analyzed.

Dem'yanov, A. Yu.; Dinariev, O. Yu.; Ivanov, E. N.

2012-11-01

354

Gas-Phase Reactions of Halogen Species of Atmospheric Importance.  

NASA Astrophysics Data System (ADS)

Available from UMI in association with The British Library. Requires signed TDF. A low-pressure discharge-flow technique, with various optical detection methods, has been used to determine bimolecular rate coefficients for a number of reactions in the gas-phase between OH radicals and organic halogen -containing molecules and between NO_3 radicals and the iodine species I_2 and I. These experiments have shown that: (i) the reaction of methyl iodide with OH accounts for approximately 2% of the removal of CH_3I from the troposphere as compared with photolysis; (ii) abstraction of I-atoms from a C-I bond by OH is probable in the gas -phase; (iii) the halogen-containing anaesthetic substances halothane CF_3CCl BrH, enflurane CF_2HOCF _2CFClH, isoflurane CF_2HOCClHCF _3 and sevoflurane (CF_3) _2CHOCFH_2 have significantly shorter tropospheric lifetimes than the fully halogenated CFCs and halons because of reaction with the OH radical and are thus unlikely to be transported up to the stratosphere where they could contribute to the depletion of ozone. Data obtained for reactions between OH and some 'CFC alternatives' along with measurements of the integrated absorption cross -sections of the compounds in the spectral region 800-1200 cm^{-1} were used to calculate ozone depletion potentials (ODP) and greenhouse warming potentials relative to CFCl_3 for each compound. The study of the reactions between OH and CF_3CFBrH and CF _2BrH was used to provide a useful first estimate of the environmental acceptability of these compounds in the context of their possible use as replacements for the conventional CFCs. A method was developed to provide a first estimate of the ODP of a halogenated alkane without use of a complicated (and expensive) computer modeling scheme. A reaction between molecular iodine and the nitrate radical in the gas-phase was discovered and the kinetics of this reaction have been studied. No temperature or pressure dependence was observed for the rate of reaction, the rate constant of which was found to be (1.5 +/- 0.5) times 10 ^{-12}cm^{ -3}molecule^{-1}s ^{-1}. The reaction between I and NO_3 was found to occur at a rate of about 60% of the hard-sphere collision frequency for the two species. The rate constant for reaction between I and NO_3 was found to be (4.5 +/- 1.9) times 10^{-10}cm^3 molecule^{-1}s ^{-1}. An upper limit for the heat of formation of IONO_2 of (21 +/- 3) kJmol^ {-1} was also derived. (Abstract shortened by UMI.).

Heard, Anne C.

355

Infrared spectra of dimethylphenanthrenes in the gas phase.  

PubMed

Infrared spectra of atmospherically and astronomically important dimethylphenanthrenes (DMPs), namely 1,9-DMP, 2,4-DMP, and 3,9-DMP, were recorded in the gas phase from 400 to 4000 cm(-1) with a resolution of 0.5 cm(-1) at 110 °C using a 7.2 m gas cell. DFT calculations at the B3LYP/6-311G** level were carried out to get the harmonic and anharmonic frequencies and their corresponding intensities for the assignment of the observed bands. However, spectral assignments could not be made unambiguously using anharmonic or selectively scaled harmonic frequencies. Therefore, the scaled quantum mechanical (SQM) force field analysis method was adopted to achieve more accurate assignments. In this method force fields instead of frequencies were scaled. The cartesian force field matrix obtained from the gaussian calculations was converted to a nonredundant local coordinate force field matrix and then the force fields were scaled to match experimental frequencies in a consistent manner using a modified version of the UMAT program of the QCPE package. Potential energy distributions (PEDs) of the normal modes in terms of nonredundant local coordinates obtained from these calculations helped us derive the nature of the vibration at each frequency. The intensity of observed bands in the experimental spectra was calculated using estimated vapor pressures of the DMPs. An error analysis of the mean deviation between experimental and calculated intensities reveal that the observed methyl C-H stretching intensity deviates more compared to the aromatic C-H and non C-H stretching bands. PMID:21797244

Das, Prasanta; Arunan, E; Das, Puspendu K

2011-07-28

356

Heterogeneous critical nucleation on a completely wettable substrate.  

PubMed

Heterogeneous nucleation of a new bulk phase on a flat substrate can be associated with the surface phase transition called wetting transition. When this bulk heterogeneous nucleation occurs on a completely wettable flat substrate with a zero contact angle, the classical nucleation theory predicts that the free-energy barrier of nucleation vanishes. In fact, there always exists a critical nucleus and a free-energy barrier as the first-order prewetting transition will occur even when the contact angle is zero. Furthermore, the critical nucleus changes its character from the critical nucleus of surface phase transition below bulk coexistence (undersaturation) to the critical nucleus of bulk heterogeneous nucleation above the coexistence (oversaturation) when it crosses the coexistence. Recently, Sear [J. Chem. Phys. 129, 164510 (2008)] has shown, by a direct numerical calculation of nucleation rate, that the nucleus does not notice this change when it crosses the coexistence. In our work, the morphology and the work of formation of critical nucleus on a completely wettable substrate are re-examined across the coexistence using the interface-displacement model. Indeed, the morphology and the work of formation changes continuously at the coexistence. Our results support the prediction of Sear and will rekindle the interest on heterogeneous nucleation on a completely wettable substrate. PMID:21702578

Iwamatsu, Masao

2011-06-21

357

Determination of gas-liquid partition coefficients by automatic equilibrium headspace-gas chromatography utilizing the phase ratio variation method  

Microsoft Academic Search

The new phase ratio variation method is described which represents a convenient way for the determination of gas-liquid partition coefficients for practical purposes, utilizing equilibrium headspace-gas chromatography (EHS-GC). This method is based on the relationship between reciprocal peak area and the phase ratio in the vial containing the sample solution; it involves regression analysis of the EHS-GC measurements of a

L. S. Ettre; C. Welter; B. Kolb

1993-01-01

358

Radio-frequency glow discharges in methane gas: modelling of the gas-phase physics and chemistry  

Microsoft Academic Search

A methane discharge fluid model is developed, and subsequently combined with a simple gas-phase chemical kinetics model. The aim is to provide better understanding of the charged species dynamics, and their interaction with the gas-phase kinetics in a CH4 plasma. Swarm data are used as input in the fluid model, which predicts the ion and electron densities, electric fields and

E. Gogolides; C. Buteau; A. Rhallabi; G. Turban

1994-01-01

359

Nucleation in n-alkanes: A density-functional approach  

NASA Astrophysics Data System (ADS)

A density-functional theory for a polyatomic system is applied to gas-liquid nucleation in n-butane and n-heptane, employing an interaction site model and the rotational isomeric state approximation. Effects of chain length and flexibility on equilibrium properties and nucleation are discussed. It is shown that the n-alkane systems cannot be well approximated by a system with the spherically symmetric Lennard-Jones potential, giving a nucleation rate scaled by the classical rate smaller by six orders of magnitude.

Seok, Chaok; Oxtoby, David W.

1998-11-01

360

Coupling a stochastic soot population balance to gas-phase chemistry using operator splitting  

Microsoft Academic Search

The feasibility of coupling a stochastic soot algorithm to a deterministic gas-phase chemistry solver is investigated for homogeneous combusting systems. A second-order splitting technique was used to decouple the particle population and gas phase in order to solve. A numerical convergence study is presented that demonstrates convergence with splitting step size and particle count for a batch reactor and a

Matthew Celnik; Robert Patterson; Markus Kraft; Wolfgang Wagner

2007-01-01

361

Theoretical Analysis of Oscillatory Burning of Homogeneous Solid Propellant Including Non-Steady Gas Phase Effects  

Microsoft Academic Search

This paper presents a theoretical analysis of the linear burning response of an homogeneous solid propellant to pressure fluctuations. The major contribution of this work is to take into account the non-steady effects of the gas phase in a systematic way and thus, eliminate the strong limitation of previous analysis in which the gas-phase is assumed to respond in a

PAUL CLAVIN; DAVID LAZIMI

1992-01-01

362

DEMONSTRATION OF FUEL CELLS TO RECOVER ENERGY FROM LANDFILL GAS: PHASE II. PRETREATMENT SYSTEM PERFORMANCE MEASUREMENT  

EPA Science Inventory

The report describes-Phase II of a demonstration of the utilization of commercial phosphoric acid fuel cells to recover energy from landfill gas. his phase consisted primarily of the construction and testing of a Gas Pretreatment Unit (GPU) whose function is to remove those impur...

363

DEMONSTRATION OF FUEL CELLS TO RECOVER ENERGY FROM LANDFILL GAS: PHASE II. PRETREATMENT SYSTEM PERFORMANCE MEASUREMENT  

EPA Science Inventory

The report describes Phase II of a demonstration of the utilization of commercial phosphoric acid fuel cells to recover energy from landfill gas. This phase consisted primarily of the construction and testing of a Gas Pretreatment Unit (GPU) whose function is to remove those impu...

364

Partitioning of phthalates among the gas phase, airborne particles and settled dust in indoor environments  

Microsoft Academic Search

A critical evaluation of human exposure to phthalate esters in indoor environments requires the determination of their distribution among the gas phase, airborne particles and settled dust. If sorption from the gas phase is the dominant mechanism whereby a given phthalate is associated with both airborne particles and settled dust, there should be a predictable relationship between its particle and

Charles J. Weschler; Tunga Salthammer; Hermann Fromme

2008-01-01

365

Gas phase and catalytic ignition of methane and ethane in air over platinum  

Microsoft Academic Search

The ignition of gas phase combustion by heated catalytically active surface involves dynamic coupling of transport processes and chemical kinetics. Researchers have observed that surfaces with high catalytic activity require higher surface temperatures to ignite gas phase combustion, compared to nonactive surfaces, and that this difference is greatest for a stoichiometric mixture. In this paper an analysis of the ignition

Timothy A. Griffin; Lisa D. Pfefferle

1990-01-01

366

Kinetics and growth mechanism of gallium arsenide crystals in gas-phase epitaxy  

Microsoft Academic Search

In conclusion, we note that gallium arsenide itself is the material with which the physicochemical and crystallophysical fundamentals of gas-phase epitaxy are presently being developed. It is hoped that the basic principles or crystal growth in gas-phase systems discovered in gallium arsenide will prove sufficiently general to be applied to other analogous systems.

L. G. Lavrent'eva

1980-01-01

367

Pt(111) reconstruction induced by enhanced Pt gas-phase chemical potential  

Microsoft Academic Search

Large terraces of the Pt(111) surface are shown to reconstruct in the presence of a Pt gas- phase environment down to 400 K. The lifting of the reconstruction in the absence of the Pt gas-phase is kinetically hindered up to 700 K. The main features of the reconstruction network as seen by the scanning tunneling microscope correspond to those inferred

Michael Bott; Michael Hohage; Thomas Michely; George Comsa

1993-01-01

368

Gas-phase coupling of reactive surfaces by oscillating reactant clouds  

Microsoft Academic Search

The collective, global behavior of a heterogeneous catalytic system depends on the effective communication of local reactivity variations to distant points in the system. One particularly efficient mode of communication occurs via partial pressure fluctuations in the gas-phase above the reactive surface. Although gas-phase communication has been implicated in a number of heterogeneous systems, the details of this coupling mechanism

Daniel Bilbao; Jochen Lauterbach

2010-01-01

369

DETERMINATION OF GAS-PHASE DIMETHYL SULFATE AND MONOMETHYL HYDROGEN SULFATE  

EPA Science Inventory

Analytical techniques have been developed for the collection and determination of gas phase dimethyl sulfate and monomethyl sulfuric acid in the flue lines and plumes of power plants and in the ambient atmosphere. The techniques involve the collection of the gas phase species in ...

370

Non-local thermodynamic equilibrium dust nucleation in subsaturated vapours  

NASA Astrophysics Data System (ADS)

We use the kinetic theory of nucleation to explore the properties of dust nucleation in subsaturated vapours. Due to radiation losses, the subcritical clusters have a smaller temperature compared to their vapour. This alters the dynamical balance between the attachment and detachment of monomers, allowing for stable nucleation of grains in vapours that are subsaturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n - T phase diagram. The nucleation in the non-local thermodynamic equilibrium (non-LTE) regions is likely to be at much slower rate than in the supersaturated vapours. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early Universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium.

Lazzati, Davide

2008-02-01

371

Gas-phase conformational and energetic properties of deprotonated dinucleotides  

NASA Astrophysics Data System (ADS)

Ion mobility experiments and molecular modeling calculations were used to investigate the gas-phase conformations and folding energetics of 16 deprotonated dinucleotides. [M H]^- ions were formed by MALDI and their collision cross-sections measured in helium using ion mobility based techniques. Cross-sections of theoretical structures, generated by molecular mechanics/dynamics calculations, were compared to the experimental values for conformational identification of the dinucleotides. Temperature dependent measurements and kinetic theory were also used to obtain energetic and dynamic data concerning the folding properties of the dinucleotides. Three distinct families of conformations, with significantly different collision cross-sections, were identified: a “stacked” family in which the two nucleobases stack; an “H-bonded” family in which the two nucleobases stay in the same plane and are hydrogen-bonded to each other; and an “open” family in which the two nucleobases are separated from each other. At temperatures geq 300 K these conformers rapidly interconvert in most systems, but they can be separated and individually observed in the lower temperature (80 200 K) experiments. The types and relative amounts of each conformer observed, and the temperature at which they can be separated, are base and sequence dependent. Theoretical modeling of the temperature-dependent data was used to determine isomerization barrier heights between the various conformers and yielded values between 0.8 12.9 kcal/mol, depending on the dinucleotide.

Gidden, J.; Bowers, M. T.

2002-09-01

372

Gas-phase ion-molecule reactions in organophosphorus esters  

PubMed

Self-condensation ion-molecule reactions of trimethyl phosphite, triethyl phosphite, dimethyl phosphonate, trimethyl phosphate and 2, 2-dichlorovinyl dimethyl phosphate (dichlorvos) were investigated by ion trap mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry. Reaction paths for the main processes observed were elucidated by parent ion selection and for reaction times up to 500 ms. In parallel, high-resolution measurements were performed in order to determine the composition of the principal ions. Among the compounds under examination, trimethyl phosphite and triethyl phosphite mainly give [M + H](+) and [M + (RO)(2)P](+) (R = CH(3), C(2)H(5)) adduct ions, whereas trimethyl phosphate and dimethyl phosphonate display [2M + H](+) ions, as the only abundant products, formed by reaction of [M + H](+) and M. 2,2-Dichlorovinyl dimethyl phosphate mostly shows fragmentation processes. The reaction patterns of the compounds examined were related to their different structural features. Gas-phase basicities of the phosphoryl compounds were also determined or re-examined. Copyright 1999 John Wiley & Sons, Ltd. PMID:10587624

Gal; Herreros; Maria; Operti; Pettigiani; Rabezzana; Vaglio

1999-12-01

373

Isomeric alkyl cation/arene complexes in the gas phase.  

PubMed

The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions. PMID:12740856

Filippi, Antonello; Roselli, Graziella; Renzi, Gabriele; Grandinetti, Felice; Speranza, Maurizio

2003-05-01

374

Gas-phase chemistry of alkylcarbonate anions and radicals  

NASA Astrophysics Data System (ADS)

Alkylcarbonate anions and radicals ROCOO-/ (R = H, CH3, C2H5, i-C3H7, and t-C4H9) are investigated in the gas phase by means of mass spectrometry and ab initio calculations. Structural parameters and energies are obtained at the MP2/6-311++G(3df,3pd)//MP2/6-311++G(d,p) level of theoryE Standard enthalpies of formation for the anions and radicals are determined via atomization energies and isodesmic reactions using the CBS-Q method. Further, alkylcarbonate anions are probed by metastable ion and collisional activation experiments, and the chemistry of the neutral radicals is investigated by charge-reversal and neutralization-reionization mass spectrometry. Although decarboxylation dominates the unimolecular reactivity of the species for both charge states, some other interesting features are observed, particularly for the anions, such as the formation of the CO3- radical anion or the presence of ionic fragments formed via hydrogen atom transfer.

Soldi-Lose, Héloïse; Schröder, Detlef; Schwarz, Helmut

2008-02-01

375

Conformational analysis of octopamine and synephrine in the gas phase.  

PubMed

Four and six conformers of the neurotransmitters octopamine and synephrine, respectively, have been identified in the gas phase using a laser ablation device in combination with a molecular-beam Fourier transform microwave spectrometer operating in the 4-10 GHz frequency range. The identification of all of the conformers was based on a comparison of the experimental rotational and (14)N quadrupole coupling constants with those predicted by ab initio calculations, as well as the relative values of their electric dipole moment components. The conformational preferences have been rationalized in terms of the various intramolecular forces operating in the different conformers of the studied molecules. All observed species are characterized by an intramolecular hydrogen bond of the type O-H···N established in the side chain of the neurotransmitters, which adopts an extended disposition in their most stable forms. For conformers with a folded side chain, an extra N-H···? hydrogen-bond-type interaction is established between the amino group and the ? system of the aromatic ring. PMID:23675821

Cabezas, Carlos; Simão, Alcides; Bermúdez, Celina; Varela, Marcelino; Peña, Isabel; Mata, Santiago; Fausto, Rui; Alonso, José Luis

2013-05-30

376

Gas phase effects on laser ignition of XM39  

NASA Astrophysics Data System (ADS)

A CO2 laser was used to investigate the effects of gas phase reactions during radiant heating of XM39 in a closed bomb at low loading density. Times to detect emission of pressurization in N2 or O2 are about the same. At flux levels (cal/(sq cm)-s) between 61 and 112, there is a minimum partial oxygen pressure required for flamespreading. At lower partial pressures, the difference between effects of O2 and N2 on the XM39 response to laser radiation is small. At higher partial pressures, the presence of O2 leads to increased pressurization rates. At one atm this minimum occurs at approximately .2 atms. Results in N2 indicate that the minimum value is lower at higher initial pressures. Predictions of a heterogeneous radiant ignition model were inconsistent with ignition delays derived from pressure measurements. The validity of the model for ignition in closed bombs may be questionable, but part of the problem may be attributed to difficulties in estimating the rate constant of the exothermic heterogeneous reaction needed for the model calculations.

Cohen, Art; Beyer, Richard; Kotlar, A.; Newberry, J.

1993-11-01

377

Isotope exchange for gas-phase acetic acid and ethanol at aqueous interfaces: A study of surface reactions  

SciTech Connect

Isotope exchange for deuterated gas-phase acetic acid and ethanol in contact with water (H{sub 2}O) droplets was studied using a droplet train apparatus. In these experiments, the gas-phase species interacts with liquid droplets and the loss of the species is monitored. The loss of the species may be due to the entry of the molecules into the bulk or to a reaction of the species at the gas-liquid interface, in this case isotope exchange. Studies were conducted as a function of pH in the range 0--14, droplet temperature in the range 291--263 K and gas-liquid interaction time in the range 2--15 ms. For deuterated acetic acid the isotope exchange probability with water molecules at the interface is near unity. On the other hand, isotope exchange probability for ethanol with surface water molecules at the interface is near unity. On the other hand, isotope exchange probability for ethanol with surface water molecules at pH 7 is much smaller, ranging from 0.033 at 263 K to 0.051 at 291 K. Ethanol isotope exchange is both acid and base catalyzed. The exchange probability therefore increases both toward low and high pH and levels off to a plateau at pH 2 and 12, respectively. The maximum value of the isotope exchange probability at the plateau is significant less than 1. It ranges between 0.14 and 0.18 with no clear trend in temperature. Results are explained in terms of a kinetic model in which it is assumed that the surface-adsorbed ethanol molecules are distributed between two distinct forms: a weakly adsorbed state and a partially solvated state. Only the partially solvated molecules can interact with the near-surface ions in the interior of the liquid. A finite rate of entering the partially solvated state is responsible for the observed plateaus in isotope exchange at high and low pH. Parameters describing the gas uptake and isotope exchange processes are examined using two models to describe the surface species: surface nucleation and Gibbs surface excess.

Shi, Q.; Li, Y.Q.; Davidovits, P. [Boston Coll., Chestnut Hill, MA (United States); Jayne, J.T.; Worsnop, D.R.; Mozurkewich, M.; Kolb, C.E. [Aerodyne Research, Inc., Billerica, MA (United States). Center for Aerosol and Cloud Chemistry

1999-04-01

378

Importance of the gas phase role to the prediction of energetic material behavior: An experimental study  

SciTech Connect

Various thermal (radiative, conductive, and convective) initiation experiments are performed to demonstrate the importance of the gas phase role in combustion modeling of energetic materials (EM). A previously published condensed phase model that includes a predicted critical irradiance above which ignition is not possible is compared to experimental laser ignition results for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 2,4,6-trinitrotoluene (TNT). Experimental results conflict with the predicted critical irradiance concept. The failure of the model is believed to result from a misconception about the role of the gas phase in the ignition process of energetic materials. The model assumes that ignition occurs at the surface and that evolution of gases inhibits ignition. High speed video of laser ignition, oven cook-off and hot wire ignition experiments captures the ignition of HMX and TNT in the gas phase. A laser ignition gap test is performed to further evaluate the effect of gas phase laser absorption and gas phase disruption on the ignition process. Results indicate that gas phase absorption of the laser energy is probably not the primary factor governing the gas phase ignition observations. It is discovered that a critical gap between an HMX pellet and a salt window of 6 mm{+-}0.4 mm exists below which ignition by CO{sub 2} laser is not possible at the tested irradiances of 29 W/cm{sup 2} and 38 W/cm{sup 2} for HMX ignition. These observations demonstrate that a significant disruption of the gas phase, in certain scenarios, will inhibit ignition, independent of any condensed phase processes. These results underscore the importance of gas phase processes and illustrate that conditions can exist where simple condensed phase models are inadequate to accurately predict the behavior of energetic materials.

Ali, A.N.; Son, S.F.; Asay, B.W.; Sander, R.K. [Los Alamos National Laboratory, MS C920, Los Alamos, New Mexico 87544 (United States)

2005-03-15

379

Nanoparticle films made by gas phase synthesis: comparison of various techniques and sensor applications  

Microsoft Academic Search

Nanoparticles can be generated by several different gas phase methods, such as gas deposition, laser-assisted chemical vapor deposition, and laser ablation. Some of the most important aspects - such as size-distribution, structure, and chemical composition of the generated nanoparticles (specifically W and WO3) - are presented and compared. WO3 nanoparticle films were deposited by an advanced gas deposition technique and

Peter Heszler; L. F. Reyes; Anders Hoel; Lars Landström; V. Lantto; Claes-Göran Granqvist

2003-01-01

380

Phase-field modeling of gas bubbles and thermal conductivity evolution in nuclear fuels  

Microsoft Academic Search

A phase-field model was developed to simulate the accumulation and transport of fission products and the evolution of gas bubble microstructures in nuclear fuels. The model takes into account the generation of gas atoms and vacancies, and the elastic interaction between diffusive species and defects as well as the inhomogeneity of elasticity and diffusivity. The simulations show that gas bubble

Shenyang Y. Hu; Charles H. Henager Jr.; Howard L. Heinisch; Marius Stan; Michael I. Baskes; Steven M. Valone

2009-01-01

381

Phase equilibrium of gas hydrate: Implications for the formation of hydrate in the deep sea floor  

Microsoft Academic Search

We calculate the solubility of methane gas over a range of pressure and temperature. The gas is dissolved in liquid water, which coexists with free gas at high temperature or solid hydrate at low temperature and high pressure. We show that solubility is significantly altered by the presence or absence of the hydrate phase. When hydrate is absent at high

Olga Ye. Zatsepina; Bruce A. Buffett

1997-01-01

382

On the ice nucleation spectrum  

NASA Astrophysics Data System (ADS)

This work presents a novel formulation of the ice nucleation spectrum, i.e. the function relating the ice crystal concentration to cloud formation conditions and aerosol properties. The new formulation is physically-based and explicitly accounts for the dependency of the ice crystal concentration on temperature, supersaturation, cooling rate, and particle size, surface area and composition. This is achieved by introducing the concepts of ice nucleation coefficient (the number of ice germs present in a particle) and nucleation probability dispersion function (the distribution of ice nucleation coefficients within the aerosol population). The new formulation is used to generate ice nucleation parameterizations for the homogeneous freezing of cloud droplets and the heterogeneous deposition ice nucleation on dust and soot ice nuclei. For homogeneous freezing, it was found that by increasing the dispersion in the droplet volume distribution the fraction of supercooled droplets in the population increases. For heterogeneous ice nucleation the new formulation consistently describes singular and stochastic behavior within a single framework. Using a fundamentally stochastic approach, both cooling rate independence and constancy of the ice nucleation fraction over time, features typically associated with singular behavior, were reproduced. Analysis of the temporal dependency of the ice nucleation spectrum suggested that experimental methods that measure the ice nucleation fraction over few seconds would tend to underestimate the ice nuclei concentration. It is shown that inferring the aerosol heterogeneous ice nucleation properties from measurements of the onset supersaturation and temperature may carry significant error as the variability in ice nucleation properties within the aerosol population is not accounted for. This work provides a simple and rigorous ice nucleation framework where theoretical predictions, laboratory measurements and field campaign data can be reconciled, and that is suitable for application in atmospheric modeling studies.

Barahona, D.

2012-04-01

383

NUMERICAL SIMULATION OF CHOKED GAS-SOLID TWO-PHASE FLOW WITH HEAT TRANSFER IN PIPES  

Microsoft Academic Search

A mathematical model for choked gas-particle two-phase flow in pipes is presented. The model takes into account the momentum and heat transfer between the gas and the particle phases. The wall surface roughness and the coupling effect are also considered. In addition effects of loading coefficient, initial slip velocity coefficient, wall roughness and particle diameter on the choked of gas-solid

Mofreh H. Hamed

2006-01-01

384

Silanetriols in the gas phase: single molecules vs. hydrogen-bonded dimers.  

PubMed

The first gas phase structure of a silanetriol, tert-butylsilane-triol [(t)BuSi(OH)(3)], determined by gas electron diffraction (GED), is reported. Quantum chemical calculations have been performed to elucidate potential intermolecular interactions between silanetriol molecules in the gas phase. The results are set into contrast to solid state structures of (t)BuSi(OH)(3) and related compounds. PMID:22089403

Spirk, Stefan; Berger, Raphael J F; Reuter, Christian G; Pietschnig, Rudolf; Mitzel, Norbert W

2011-11-17

385

Heterogeneous nucleation in the low-barrier regime.  

PubMed

In simulations of the two-dimensional Ising model, we examine heterogeneous nucleation induced by a small impurity consisting of a line of l fixed spins. As l increases, we identify a limit of stability beyond which the metastable phase is not defined. We evaluate the free energy barrier for nucleation of the stable phase and show that, contrary to expectation, the barrier does not vanish on approach to the limit of stability. We also demonstrate that our values for the height of the barrier yield predictions for the nucleation time (from transition state theory) and the size of the critical cluster (from the nucleation theorem) that are in excellent agreement with direct measurements, even near the limit of stability. PMID:23679429

Scheifele, Benjamin; Saika-Voivod, Ivan; Bowles, Richard K; Poole, Peter H

2013-04-26

386

Heterogeneous nucleation in the low-barrier regime  

NASA Astrophysics Data System (ADS)

In simulations of the two-dimensional Ising model, we examine heterogeneous nucleation induced by a small impurity consisting of a line of l fixed spins. As l increases, we identify a limit of stability beyond which the metastable phase is not defined. We evaluate the free energy barrier for nucleation of the stable phase and show that, contrary to expectation, the barrier does not vanish on approach to the limit of stability. We also demonstrate that our values for the height of the barrier yield predictions for the nucleation time (from transition state theory) and the size of the critical cluster (from the nucleation theorem) that are in excellent agreement with direct measurements, even near the limit of stability.

Scheifele, Benjamin; Saika-Voivod, Ivan; Bowles, Richard K.; Poole, Peter H.

2013-04-01

387

Gas-phase interactions of organotin compounds with glycine.  

PubMed

Gas-phase interactions of organotins with glycine have been studied by combining mass spectrometry experiments and quantum calculations. Positive-ion electrospray spectra show that the interaction of di- and tri-organotins with glycine results in the formation of [(R)2Sn(Gly)-H](+) and [(R)3Sn(Gly)](+) ions, respectively. Di-organotin complexes appear much more reactive than those involving tri-organotins. (MS/MS) spectra of the [(R)3Sn(Gly)](+) ions are indeed simple and only show elimination of intact glycine, generating the [(R)3Sn](+) carbocation. On the other hand, MS/MS spectra of [(R)2Sn(Gly)-H](+) complexes are characterized by numerous fragmentation processes. Six of them, associated with elimination of H2O, CO, H2O?+?CO and formation of [(R)2SnOH](+) (-57?u),[(R)2SnNH2](+) (-58?u) and [(R)2SnH](+) (-73?u), are systematically observed. Use of labeled glycines notably concludes that the hydrogen atoms eliminated in water and H2O?+?CO are labile hydrogens. A similar conclusion can be made for hydrogens of [(R2)SnOH](+) and [(R2)SnNH2](+) ions. Interestingly, formation [(R)2SnH](+) ions is characterized by a migration of one the ? hydrogen of glycine onto the metallic center. Finally, several dissociation routes are observed and are characteristic of a given organic substituent. Calculations indicated that the interaction between organotins and glycine is mostly electrostatic. For [(R)2Sn(Gly)-H](+) complexes, a preferable bidentate interaction of the type ?(2)-O,NH2 is observed, similar to that encountered for other metal ions. [(R)3Sn](+) ions strongly stabilize the zwitterionic form of glycine, which is practically degenerate with respect to neutral glycine. In addition, the interconversion between both forms is almost barrierless. Suitable mechanisms are proposed in order to account for the most relevant fragmentation processes. PMID:23832935

Latrous, Latifa; Tortajada, Jeanine; Haldys, Violette; Léon, Emmanuelle; Correia, Catarina; Salpin, Jean-Yves

2013-07-01

388

Infrared spectroscopy of ionized corannulene in the gas phase  

NASA Astrophysics Data System (ADS)

The gas-phase infrared spectra of radical cationic and protonated corannulene were recorded by infrared multiple-photon dissociation (IRMPD) spectroscopy using the IR free electron laser for infrared experiments. Electrospray ionization was used to generate protonated corannulene and an IRMPD spectrum was recorded in a Fourier-transform ion cyclotron resonance mass spectrometer monitoring H-loss as a function of IR frequency. The radical cation was produced by 193-nm UV photoionization of the vapor of corannulene in a 3D quadrupole trap and IR irradiation produces H, H2, and C2Hx losses. Summing the spectral response of the three fragmentation channels yields the IRMPD spectrum of the radical cation. The spectra were analyzed with the aid of quantum-chemical calculations carried out at various levels of theory. The good agreement of theoretical and experimental spectra for protonated corannulene indicates that protonation occurs on one of the peripheral C-atoms, forming an sp3 hybridized carbon. The spectrum of the radical cation was examined taking into account distortions of the C5v geometry induced by the Jahn-Teller effect as a consequence of the degenerate 2E1 ground electronic state. As indicated by the calculations, the five equivalent Cs minima are separated by marginal barriers, giving rise to a dynamically distorted system. Although in general the character of the various computed vibrational bands appears to be in order, only a qualitative match to the experimental spectrum is found. Along with a general redshift of the calculated frequencies, the IR intensities of modes in the 1000-1250 cm-1 region show the largest discrepancy with the harmonic predictions. In addition to CH ``in-plane'' bending vibrations, these modes also exhibit substantial deformation of the pentagonal inner ring, which may relate directly to the vibronic interaction in the radical cation.

Galué, Héctor Alvaro; Rice, Corey A.; Steill, Jeffrey D.; Oomens, Jos

2011-02-01

389

The Gas-Phase Deuterium Fractionation of Formaldehyde  

NASA Astrophysics Data System (ADS)

The dominant mechanism for the deuteration of formaldehyde in the gas phase of low-temperature interstellar cloud cores occurs via reaction with the deuterating ions H2D+, HD+2, and D+3. Until now, it has been assumed that deuteration leads to an ion that, on recombination with electrons, can produce a deuterated neutral species with a statistical branching fraction. Quantum chemical calculations reported here, however, show an entirely different picture, in which the deuteration of formaldehyde leads to the molecular ion H2COD+, where the deuterium binds only on the oxygen side of the molecule. The structure is quite stable, while an alternative structure, H2DCO+, cannot be produced in a straightforward manner. Dissociative recombination of H2COD+ to reproduce a formaldehyde structure then removes the deuteration if the dissociation is direct, i.e., it occurs without change of structure. There are several possible indirect mechanisms by which dissociative recombination can lead to HDCO, however. For example, if the direct products are HCOD+H, it is possible that subsequent isomerization to HDCO can occur, although this involved process is unlikely. Another possibility is isomerization during the actual dissociation of the H2COD intermediate. Models of deuterium fractionation in which dissociative recombination is predominantly direct are presented, and it is found that the deuterium fractionation of formaldehyde to form both HDCO and D2CO can still occur via other mechanisms, although with less efficiency than previously obtained. If the dissociative recombination is half indirect, however, then we can recover the previously calculated efficiency.

Osamura, Yoshihiro; Roberts, Helen; Herbst, Eric

2005-03-01

390

Study on measures to improve gas-liquid phase mixing in a multiphase pump impeller under high gas void fraction  

NASA Astrophysics Data System (ADS)

Rotodynamic multiphase pump can transport crude gas-liquid mixture produced from oil well, and is regarded as the good choice of oil-gas multiphase transportation in offshore product system, for its advantages that compact structure, large flow rate, not sensitive to solid particles in the fluid. However, it is prone to bring about gas-liquid separation within the impeller under high gas void fraction. To solve the problem, this paper presents several measures to break gas packet and inhibit gas-liquid separation, such as, depositing the short blades, opening holes at the blades where gas packets gather, using T-shaped blades, etc. Then, CFD software was used to simulate the flow fields which were added measures to inhibit gas-liquid separation. The results show that streamlines in three new impellers distribute more evenly than in original impeller, the gas-liquid two phases mixed degree was improved, and the gas-liquid separation was inhibited to some extent. However, adding the short blades and using T-blade impeller failed to improve the differential pressure of impellers. So the placement and the geometrical parameters of the measures inhibiting gas-liquid separation should be further optimized.

Zhang, J. Y.; Zhu, H. W.; Ding, K.; Qiang, R.

2012-11-01

391

Substrate orientation effects on the nucleation and growth of the M{sub n+1}AX{sub n} phase TiAlC  

Microsoft Academic Search

The M{sub n+1}AX{sub n} (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third ''A-group'' element. The effect of substrate orientation on the growth of TiAlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (1010), and (1102) crystallographic planes. Characterization of

Mark D. Tucker; Mathew C. Guenette; Marcela M. M. Bilek; David R. McKenzie; Per O. A. Persson; Johanna Rosen

2011-01-01

392

Protein Crystal Nucleation and Growth  

NASA Astrophysics Data System (ADS)

We have developed a microfluidic emulsion based technique to determine the homogeneous and heterogeneous nucleation rates of protein crystallization under conditions of high supersaturation. We utilize the fact that the nucleation rate is constant if no crystal nucleus is formed and count the number of protein droplets with no crystals with time, which decays exponentially with decay constant inversely proportional to nucleation rate and drop volume. We report results of experiments on nucleation and growth rates of lysozyme crystallization. The emulsions are placed on a temperature gradient stage allowing simultaneous measurement of rates as a function of temperature. We routinely scan 30,000 drops in each experiment.

Akella, Sathish; Fraden, Seth

2012-02-01

393

CO2 Capture from Flue Gas by Phase Transitional Absorption  

SciTech Connect

A novel absorption process called Phase Transitional Absorption was invented. What is the Phase Transitional Absorption? Phase Transitional Absorption is a two or multi phase absorption system, CO{sub 2} rich phase and CO{sub 2} lean phase. During Absorption, CO{sub 2} is accumulated in CO{sub 2} rich phase. After separating the two phases, CO{sub 2} rich phase is forward to regeneration. After regeneration, the regenerated CO{sub 2} rich phase combines CO{sub 2} lean phase to form absorbent again to complete the cycle. The advantage for Phase Transitional Absorption is obvious, significantly saving on regeneration energy. Because CO{sub 2} lean phase was separated before regeneration, only CO{sub 2} rich phase was forward to regeneration. The absorption system we developed has the features of high absorption rate, high loading and working capacity, low corrosion, low regeneration heat, no toxic to environment, etc. The process evaluation shows that our process is able to save 80% energy cost by comparing with MEA process.

Liang Hu

2009-06-30

394

Three-phase measurements of oil and gas trapping in sand packs  

NASA Astrophysics Data System (ADS)

We measure the trapped saturations of oil and gas as a function of initial saturation in water-wet sand packs. We start with a water-saturated column and inject octane (oil), while water and oil are produced from the bottom. Once water production has ceased, air (gas) then enters from the top, allowing oil and gas to drain under gravity for different times. Finally water is then injected from the bottom to trap both oil and gas. The columns are sliced and the fluids analyzed using gas chromatography. We find that for high initial gas saturations more gas can be trapped in the presence of oil than in a two-phase (gas/water) system. The residual gas saturation can be over 20% compared to 14% in two-phase flow [Al Mansoori SK, Iglauer S, Pentland CH, Bijeljic B, Blunt MJ. Measurements of non-wetting phase trapping applied to carbon dioxide storage. Energy Procedia 2009;1(1):3173-80]. This is unlike previous measurements on consolidated media, where the trapped gas saturation is either similar or lower to that reached in an equivalent two-phase experiment. For lower initial gas saturation, the amount of trapping follows the initial-residual trend seen in two-phase experiments. The amount of oil trapped is insensitive to initial gas saturation or the amount of gas that is trapped, again in contrast to measurements on consolidated media. More oil is trapped than would be predicted from an equivalent two-phase (oil/water) system, although the trapped saturation is never larger than the maximum reached in two-phase flow (around 11%) [Pentland CH, Al Mansoori SK, Iglauer S, Bijeljic B, Blunt MJ. Measurement of non-wetting phase trapping in sand packs. In: SPE 115697, proceedings of the SPE annual technical conference and exhibition, Denver, Colorado, USA; 21-24 September 2008]. These initially surprising results are explained in the context of oil layer stability and the competition between snap-off and piston-like advance. In two-phase systems, displacement is principally by cooperative piston-like advance with relatively little trapping, whereas in consolidated media snap-off is generally more significant. However, oil layer collapse events during three-phase waterflooding rapidly trap the oil which acts as a barrier to direct water/gas displacement, except by snap-off, leading to enhanced gas trapping.

Al Mansoori, Saleh K.; Iglauer, Stefan; Pentland, Christopher H.; Blunt, Martin J.

395

Two-phase gas bubble-liquid boundary layer flow along vertical and inclined surfaces  

SciTech Connect

The behavior of a two-phase gas bubble-liquid boundary layer along vertical and inclined porous surfaces with uniform gas injection is investigated experimentally and analytically. Using argon gas and water as the working fluids, a photographical study of the two-phase boundary layer flow has been performed for various angles of inclination ranging from 45/sup 0/ to 135/sup 0/ and gas injection rates ranging from 0.01 to 0.1 m/s. An integral method has been employed to solve the system of equations governing the two-phase motion. The effects of the gas injection rate and the angle of inclination on the growth of the boundary layer have been determined. The predicted boundary layer thickness is found to be in good agreement with the experimental results. The calculated axial liquid velocity and the void fraction in the two-phase region are also presented along with the observed flow behavior.

Cheung, F.B.; Epstein, M.

1985-01-01

396

Nucleation of nuclear bodies.  

PubMed

The nucleus is a complex organelle containing numerous highly dynamic, structurally stable domains and bodies, harboring functions that have only begun to be defined. However, the molecular mechanisms for their formation are still poorly understood. Recently it has been shown that a nuclear body can form de novo by self-organization. But little is known regarding what triggers the formation of a nuclear body and how subsequent assembly steps are orchestrated. Nuclear bodies are frequently associated with specific active gene loci that directly contribute to their formation. Both coding and noncoding RNAs can initiate the assembly of nuclear bodies with which they are physiologically associated. Thus, the formation of nuclear bodies occurs via recruitment and consequent accumulation of resident proteins in the nuclear bodies by nucleating RNA acting as a seeder. In this chapter I describe how to set up an experimental cell system to probe de novo biogenesis of a nuclear body by nucleating RNA and nuclear body components tethered on chromatin. PMID:23980018

Dundr, Miroslav

2013-01-01

397

Surftherm: A program to analyze thermochemical and kinetic data in gas-phase and surface chemical reaction mechanisms.  

National Technical Information Service (NTIS)

This report documents the Surftherm program that analyzes transport coefficient, thermochemical- and kinetic rate information in complex gas-phase and surface chemical reaction mechanisms. The program is designed for use with the Chemkin (gas-phase chemis...

M. E. Coltrin H. K. Moffat

1994-01-01

398

Tentative Method for the Calibration of Nitric Oxide, Nitrogen Dioxide, and Ozone Analyzers by Gas Phase Titration.  

National Technical Information Service (NTIS)

A detailed procedural description of a technique for the dynamic calibration of ambient air monitors for ozone, nitric oxide, and nitrogen dioxide is presented. A gas phase titration technique utilizing the rapid gas phase reaction between nitric oxide an...

K. A. Rehme B. E. Martin J. A. Hodgeson

1974-01-01

399

Homogeneous nucleation during melting and spallation  

Microsoft Academic Search

Melt and void nucleation can be described with the classical nucleation theory in similar manners, and the critical nucleus size and steady state nucleation rate are key parameters. Numerical experiments such as molecular dynamics (MD) simulations yield atomistic scale details on nucleation, and different statistical methods are available to reduce MD simulations for extracting nucleation information. We conduct MD simulations

Sheng-Nian Luo; Qi An; Timothy Germann; Li-Bo Han

2009-01-01

400

Enhancement of gas-phase diffusion in the presence of liquid  

NASA Astrophysics Data System (ADS)

Gas diffusion in porous media occurs in both the gas and liquid phases. In many instances, gas diffusion in the liquid phase is ignored. However, under many conditions, gas diffusion in the liquid phase may be more important than gas diffusion in the gas phase. Two different cases will be examined in this work. The first case is a continuous liquid path between the gas concentrations of interest modeled after Jury et al. (1984). The second case is the situation at low liquid saturation where liquid islands exist. For the first case, Jury's model can be rewritten as a ratio of the total gas diffusion in the gas and liquid phases to that just in the gas phase. The liquid diffusion coefficient is approximately 10-4 times the gas diffusion coefficient consistent with Jury et al. (1984). The ratio of total diffusion to gas-phase diffusion is then only a function of Henry's constant and the liquid saturation. For higher values of Henry's constant, such as for CO2 and O2, the effect of diffusion in the liquid phase is small except at high liquid saturations. For small values of Henry's constant, such as for some VOCs and explosive compounds, diffusion in the liquid phase dominates for low and moderate liquid saturation values. The second case is the enhancement of diffusion caused by liquid islands at low liquid saturation. Enhanced vapor diffusion across liquid islands has been observed and modeled by Webb and Ho (1999), where condensation and evaporation occur on opposite ends of the liquid island. Vapor diffusion enhancement of up to a factor of 10 has been observed. Similarly, gas can diffuse through the liquid island. For high values of Henry's constant, gas diffusion through liquid islands is negligible and can be ignored. For small values of Henry's constant, diffusion through liquid islands may be much greater than diffusion through gas, so the rate is enhanced. The work was sponsored by the Geneva International Center for Humanitarian Demining (GICHD) under the direction of Havard Bach. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

Webb, S.; Angert, A.

2003-04-01

401

The gas-phase thermal chemistry of tetralin and related model systems  

SciTech Connect

The thesis is divided into 5 papers: gas-phase thermal decomposition of tetralin; flash vacuum pyrolysis of 3-benzocycloheptenone and 1,3, 4,5-tetrahydro-2-benzothiepin-2,2-dioxide (model systems for gas-phase pyrolysis of tetralin); high-temperature gas-phase reactions of o-allylbenzyl radicals generated by flash vacuum pyrolysis of is(o-allylbenzyl) oxalate; flash vacuum pyrolysis of 1,4-diphenylbutane; and flash vacuum pyrolysis of o-allyltoluene, o-(3-butenyl)toluene and o-(pentenyl)toluene were also used.

Malandra, J.

1993-05-01

402

Calcium-containing diatomic dications in the gas phase.  

PubMed

Sputtering (ion surface bombardment) of various calcium-containing powder samples with an energetic (17 keV), high-current (16)O(-) beam has produced the diatomic dications of CaSi(2+), CaP(2+), CaF(2+), CaH(2+), CaCl(2+), CaBr(2+) and CaI(2+). These molecular gas-phase species have been identified in positive ion mass spectra at half-integer m/z values; their ion flight times through a magnetic-sector mass spectrometer were roughly 10(-5) s. Most of them appear to be novel molecular ions; the stability of the latter four (CaH(2+), CaCl(2+), CaBr(2+) and CaI(2+)) had been demonstrated in previous theoretical studies, whereas only CaF(2+) and CaBr(2+) had been observed before. Here we combine the results of our experimental search with a detailed theoretical study of the remaining three systems CaSi(2+), CaP(2+) and CaF(2+). All electronic states correlating with the first dissociation channel are characterized using high level ab initio electronic structure calculations. In their ground states, we find CaSi(2+) to be a long-lived metastable molecule, whereas CaF(2+) and CaP(2+) are thermodynamically stable, with respective equilibrium internuclear distances of 6.253, 4.740, and 5.731 a(0). CaSi(2+) has a well depth of 7116 (0.88) cm(-1) (eV) and a dissociation asymptote 7956 (0.99) cm(-1) (eV) below the ground state minimum. The dissociation energy of CaF(2+) is estimated to be 3404 (0.42) cm(-1) (eV), whereas for CaP(2+) we found 2547 (0.32) cm(-1) (eV), and a barrier height of 8118 (1.01) cm(-1) (eV). Their adiabatic double ionisation energies are 22.87, 16.91, and 17.32 eV, respectively, for the F, Si, and P containing dications. PMID:21603705

Alves, Tiago Vinicius; Hermoso, Willian; Franzreb, Klaus; Ornellas, Fernando R

2011-05-20

403

Gas phase chemistry of bis(pentamethylcyclopentadienyl)samarium  

SciTech Connect

The gas phase chemistry of bis(pentamethylcyclopentadienyl)samarium, (C{sub 5}Me{sub 5}){sub 2}Sm, was studied by Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS). Positive electron impact (EI) spectra showed the formation of (C{sub 5}Me{sub 5}){sub 2} Sm{sup +}, (C{sub 5}Me{sub 5})Sm{sup +}, and Sm{sup +}. All three ions reacted with (C{sub 5}Me{sub 5}){sub 2}Sm by charge transfer, as verified by double-resonance techniques, and (C{sub 5}Me{sub 5})Sm{sup +} also formed the (C{sub 5}Me{sub 5}){sub 3}Sm{sub 2}{sup +} ion in a condensation reaction with neutral (C{sub 5}Me{sub 5}){sub 2}Sm. The laser desorption/ionization (LDI) spectra showed, in addition to (C{sub 5}Me{sub 5}){sub 2}Sm{sup +}, (C{sub 5}Me{sub 5})Sm{sup +}, and Sm{sup +}, the formation of (C{sub 5}Me{sub 4}H)Sm{sup +} and (C{sub 5}Me{sub 4}CH{sub 2})Sm{sup +}. The latter species most probably involves a tetramethylfulvenide ligand. Access to all of the ionic species cited here could also be obtained by reacting laser-desorbed Sm{sup +} ions with pentamethylcyclopentadiene, C{sub 5}Me{sub 5}H. (C{sub 5}Me{sub 4}CH{sub 2})Sm{sup +}, (C{sub 5}Me{sub 4}H)Sm{sup +}, and (C{sub 5}Me{sub 5})Sm{sup +} were formed as primary products, and the metallocene ion (C{sub 5}Me{sub 5}){sub 2}Sm{sup +} resulted from the rapid addition of C{sub 5}Me{sub 5}H to (C{sub 5}Me{sub 4}CH{sub 2})Sm{sup +}. 34 refs., 4 figs.

Marcalo, J.; Matos, A.P. de [Instituto Technologico e Nuclear, Sacavem (Portugal); Evans, W. [Univ. of California, Irvine, CA (United States)

1996-01-09

404

The orthogonal character of stationary phases for gas chromatography.  

PubMed

A database of system constants for 32 open-tubular columns at 100 degrees C is used to identify stationary phases for obtaining a wide selectivity space in comprehensive GC. Three parameters based on the Euclidean distance (D-parameter) or vectors (d-parameter and costheta) in hyperspace are used to establish the chemical similarity and retention correlation as an inverse scale of selectivity differences. It is shown that the poly(methyloctylsiloxane) stationary phase is the best candidate for a low-selectivity stationary phase and affords a wider selectivity space when combined with a selective polar stationary phase than poly(dimethylsiloxanes). The most suitable polar stationary phases are poly(ethylene glycols) or bis(cyanopropylsiloxane-co-silarylenes and to a lesser extent poly(methyltrifluoropropylsiloxanes). No systems are truly orthogonal but angles between individual stationary phase vectors of about 75 degrees are possible by choosing the correct combination of stationary phases. PMID:18306435

Poole, Salwa K; Poole, Colin F

2008-04-01

405

Gas phase RNA and DNA ions 2. Conformational dependence of the gas-phase H\\/D exchange of nucleotide-5?-monophosphates  

Microsoft Academic Search

The conformational dependence of the gas-phase hydrogen\\/deuterium (H\\/D) exchange of nucleotide-5-monophosphate anions with\\u000a the H\\/D exchange reagent D2S is reported here. The electrospray-generated [M ? H]? anions of adenosine-5?-monophosphate, adenosine-5?-carboxylic acid, ribitol-5-phosphate, and 2-deoxy-ribitol-5-phosphate\\u000a were reacted with D2S in the gas phase. Their reactivity (adenosine-5?-monophosphate exchanged 2 of 5 labile hydrogens, adenosine-5?-carboxylic\\u000a acid exchanged 1 of 4, ribitol-5-phosphate exchanged 2

Michael A. Freitas; Alan G. Marshall

2001-01-01

406

Magnetic properties of iron-oxide passivated iron nanoparticles synthesized by a gas condensation technique  

Microsoft Academic Search

Gas phase synthesis processes involve the generation of metal atoms through various means, and the homogeneous nucleation and subsequent condensation and coagulation of nanoparticles. Inert gas condensation (IGC) is a desirable process for the synthesis of metal nanoparticles because it is a relatively simple process capable of producing large quantities of nanoparticles, and since it utilizes vacuum deposition, it offers

Colin C. Baker

2004-01-01

407

A new approach in modelling phase equilibria and gas solubility in electrolyte solutions and its applications to gas hydrates  

Microsoft Academic Search

This paper presents a new predictive model for phase equilibria and gas solubility calculations in the presence of electrolyte solutions. It treats salts as pseudo-components in an equation of state (EoS) by defining the critical properties and acentric factor for each salt. The water–salt, gas–salt and salt–salt binary interaction parameters (BIP) have been determined by using available experimental data on

Rahim Masoudi; Bahman Tohidi; Ali Danesh; Adrian C. Todd

2004-01-01

408

Dispersion and transport of gas-phase contaminants in dry porous media: effect of heterogeneity and gas velocity  

Microsoft Academic Search

The purpose of these experiments was to study the effects of physical heterogeneity and velocity on transport of gas-phase contaminants in dry porous media. Experiments were conducted at gas velocities ranging from 6 to 200 cm min?1 to examine the contributions of longitudinal molecular diffusion, hydrodynamic dispersion, and rate-limited diffusive mass transfer to solute spreading. Methane was used as a

Jarmila Popovi?ová; Mark L. Brusseau

1997-01-01

409

Nucleation of Crystals in Solution  

NASA Astrophysics Data System (ADS)

Solution crystallization is an essential part of processes in the chemical and pharmaceutical industries and a major step in physiological and pathological phenomena. Crystallization starts with nucleation and control of nucleation is crucial for the control of the number, size, perfection, polymorphism and other characteristics of the crystalline materials. Recently, there have been significant advances in the understanding of the mechanism of nucleation of crystals in solution. The most significant of these is the two-step mechanism of nucleation, according to which the crystalline nucleus appears inside pre-existing metastable clusters of size several hundred nanometers, which consist of dense liquid and are suspended in the solution. While initially proposed for protein crystals, the applicability of this mechanism has been demonstrated for small molecule organic materials, colloids, and biominerals. This mechanism helps to explain several long-standing puzzles of crystal nucleation in solution: nucleation rates which are many orders of magnitude lower than theoretical predictions, nucleation kinetic dependencies with steady or receding parts at increasing supersaturation, the role of heterogeneous substrates for polymorph selection, the significance of the dense protein liquid, and others. More importantly, this mechanism provides powerful tools for control of the nucleation process by varying the solution thermodynamic parameters so that the volume occupied by the dense liquid shrinks or expands.

Vekilov, Peter G.

2010-07-01

410

On Capillary Rise and Nucleation  

ERIC Educational Resources Information Center

A comparison of capillary rise and nucleation is presented. It is shown that both phenomena result from a balance between two competing energy factors: a volume energy and a surface energy. Such a comparison may help to introduce nucleation with a topic familiar to the students, capillary rise. (Contains 1 table and 3 figures.)

Prasad, R.

2008-01-01

411

Heterogeneous nucleation on aerosol particles  

Microsoft Academic Search

We have applied density functional theory in statistical mechanisms to study the heterogeneous nucleation of supersaturated vapors on spherical aerosol-like substrates. Our calculations reveal the inadequacy of the classical nucleation theory in describing the condensation of droplets on very small particles. The latter approach is particularly inaccurate both at high supersaturations and in the vicinity of the wetting transition. Comparisons

Kira Padilla; V. Talanquer

2001-01-01

412

On Capillary Rise and Nucleation  

ERIC Educational Resources Information Center

|A comparison of capillary rise and nucleation is presented. It is shown that both phenomena result from a balance between two competing energy factors: a volume energy and a surface energy. Such a comparison may help to introduce nucleation with a topic familiar to the students, capillary rise. (Contains 1 table and 3 figures.)|

Prasad, R.

2008-01-01

413

Microencapsulated phase-change materials as heat transfer media in gas-fluidized beds  

Microsoft Academic Search

The objective of this research is to investigate microencapsulated phase-change materials as heat-transfer media in gas-fluidized beds. The effective specific heat of an encapsulated material undergoing phase-transition is several-fold larger than the specific heat of the same material undergoing sensible heat changes. Because heat transfer coefficients in a fluidized bed increase with specific heat of the particulate phase, encapsulated phase-change

Robert C. Brown; John D. Rasberry; Scott P. Overmann

1998-01-01

414

Toward a molecular theory of homogeneous bubble nucleation in superheated liquids  

Microsoft Academic Search

Superheated liquids play an important role in various processes in industry, the laboratory and nature. Examples include hazardous vapor explosions, sonochemistry, ascent of tree sap, etc. Superheated liquids have finite lifetimes, after which they phase separate to the vapor phase by the process of bubble nucleation. This thesis describes an investigation of the molecular mechanisms of homogeneous bubble nucleation via

Sudeep Neelakantan Punnathanam

2003-01-01

415

Phase diagram of the charged lattice-gas model with two types of particles.  

PubMed

A lattice-gas model with two types of particles, a particle-dependent short-range coupling and a long-range repulsive Coulombic interaction, is introduced. The phase diagram of an isolated finite system of 129 particles is constructed using the bimodality properties of the observables' distribution. We show that this generic Hamiltonian, with couplings optimized on the properties of the atomic nucleus, exhibits a specific phase diagram including, together with the well-known liquid-gas phase transition, a segregation phase that can be assimilated to nuclear fission. PMID:20866182

Lehaut, G; Gulminelli, F; Lopez, O

2010-05-06

416

The nucleation of Fe-Rich phases on oxide films in Al11.5Si0.4Mg cast alloys  

Microsoft Academic Search

The microstructures of Al-11.5Si-0.4Mg alloys with various Fe and Mn contents have been studied to investigate the potential\\u000a influence of oxide films on the precipitation of Fe-rich phases from the liquid metal. Oxide films are incorporated into melts\\u000a by an entrainment process. This is an enfolding mechanism of incorporation. Folded oxide films in melts have two sides: the\\u000a dry unbonded

X. Cao; J. Campbell

2003-01-01

417

Chiral domains of the SmCPA phase formed by cooling the isotropic liquid or by field-induced nucleation above the clearing temperature  

Microsoft Academic Search

Three homologues belonging to a new class of achiral and asymmetric bent-core mesogens have been synthesized. The mesophase behaviour was investigated using polarizing optical microscopy, and by X-ray and electro-optical measurements. On cooling the isotropic liquid, the SmCPA phase of the dodecyloxy homologue forms a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. Surprisingly, such chiral domains could

Martin W. Schröder; Ulrike Dunemann; Wolfgang Weissflog

2004-01-01

418

Source apportionment of wintertime gas-phase and particle-phase air pollutants using organic compounds as tracers  

SciTech Connect

Two chemical mass balance receptor models are developed which can determine the source contributions to atmospheric pollutant concentrations using organic compounds as tracers. The first model uses particle-phase organic compounds to apportion the primary source contribution to atmospheric fine particulate organic carbon concentrations and fine particle mass concentrations. The second receptor model simultaneously uses both volatile gas-phase hydrocarbon and particle-phase organic compounds as tracers to determine source contributions to non-methane organic gases in the atmosphere. Both models are applied to data collected in California's San Joaquin Valley during two severe wintertime air pollution episodes. Source contributions to fine particle air quality are calculated for two urban sites, Fresno and Bakersfield, and one background site, Kern Wildlife Refuge. Primary particle emissions from hardwood combustion, softwood combustion, diesel engines, meat cooking, and gasoline-powered motor vehicles contribute on average 79% of the airborne fine particle organic compound mass at the urban sites during both episodes with smaller but still measurable contributions from fine particle road dust and natural gas combustion aerosol. Anthropogenic primary particle sources contribute less than 10% of the fine particle mass concentration at the background site. The combined gas-phase and particle-phase organic compound receptor model shows that gasoline-powered motor vehicle exhaust and gasoline vapors are the largest contributors to nonmethane organic gases concentrations followed by natural gas leakage. Smaller but statistically significant contributions to organic vapors from wood combustion, meat cooking, and diesel exhaust also are quantified.

Schauer, J.J.; Cass, G.R.

2000-05-01

419

Exhaust Gas Heat Recovery Demonstration: Phase 1 Final Report.  

National Technical Information Service (NTIS)

Recovery of some of the unavailable energy in the exhaust gases of internal combustion engines is an attractive concept for conservation of petroleum based fuels. The installation of an exhaust gas heat recovery (EGHR) system in highway transportation veh...

C. C. Love

1981-01-01

420

DETERMINATION OF CHLOROETHENES IN ENVIRONMENTAL BIOLOGICAL SAMPLES USING GAS CHROMATOGRAPHY COUPLED WITH SOLID PHASE MICRO EXTRACTION  

EPA Science Inventory

An analytical method has been developed to determine the chloroethene series, tetrachloroethene (PCE), trichloroethene (TCE),cisdichloroethene (cis-DCE) andtransdichloroethene (trans-DCE) in environmental biotreatment studies using gas chromatography coupled with a solid phase mi...